DEPR: DateOffset.apply and DateOffset.apply_index
```pytb
In [1]: import pandas as pd

In [2]: rng = pd.date_range(start="1/1/2000", periods=10, freq="T")

In [3]: pd.offsets.YearEnd()
Out[3]: <YearEnd: month=12>

In [4]: pd.offsets.YearEnd().apply_index(rng)
Out[4]:
DatetimeIndex(['2000-12-31 00:00:00', '2000-12-31 00:01:00',
               '2000-12-31 00:02:00', '2000-12-31 00:03:00',
               '2000-12-31 00:04:00', '2000-12-31 00:05:00',
               '2000-12-31 00:06:00', '2000-12-31 00:07:00',
               '2000-12-31 00:08:00', '2000-12-31 00:09:00'],
              dtype='datetime64[ns]', freq='T')

In [5]: pd.__version__
Out[5]: '1.0.4'

```

vs

```python
In [1]: import pandas as pd

In [2]: rng = pd.date_range(start="1/1/2000", periods=10, freq="T")

In [3]: pd.offsets.YearEnd().apply_index(rng)
Out[3]:
array([978220800000000000, 978220860000000000, 978220920000000000,
       978220980000000000, 978221040000000000, 978221100000000000,
       978221160000000000, 978221220000000000, 978221280000000000,
       978221340000000000])

In [4]: pd.__version__
Out[4]: '1.1.0.dev0+2021.g74538104bc'

```

---


see https://github.com/pandas-dev/pandas/pull/34579#discussion_r435465450

errors are here: https://github.com/pandas-dev/pandas/pull/33962#issuecomment-638834572

these don't need to be exposed to users and we should deprecate; it seems that in the offsets reorg some of the offsets Month? are no longer exposing these methods as well (so need to test).

```[  0.65%] ···· For parameters: <BusinessDay>
               Traceback (most recent call last):
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 1184, in main_run_server
                   main_run(run_args)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 1058, in main_run
                   result = benchmark.do_run()
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 537, in do_run
                   return self.run(*self._current_params)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 627, in run
                   samples, number = self.benchmark_timing(timer, min_repeat, max_repeat,
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 694, in benchmark_timing
                   timing = timer.timeit(number)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/timeit.py", line 177, in timeit
                   timing = self.inner(it, self.timer)
                 File "<timeit-src>", line 6, in inner
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 599, in <lambda>
                   func = lambda: self.func(*param)
                 File "/home/runner/work/pandas/pandas/asv_bench/benchmarks/arithmetic.py", line 469, in time_apply_index
                   offset.apply_index(self.rng)
                 File "pandas/_libs/tslibs/offsets.pyx", line 87, in pandas._libs.tslibs.offsets.apply_index_wraps.wrapper
                 File "pandas/_libs/tslibs/offsets.pyx", line 1397, in pandas._libs.tslibs.offsets.BusinessDay.apply_index
               AttributeError: 'PeriodIndex' object has no attribute '_addsub_int_array'
               
               For parameters: <SemiMonthEnd: day_of_month=15>
               Traceback (most recent call last):
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 1184, in main_run_server
                   main_run(run_args)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 1058, in main_run
                   result = benchmark.do_run()
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 537, in do_run
                   return self.run(*self._current_params)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 627, in run
                   samples, number = self.benchmark_timing(timer, min_repeat, max_repeat,
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 694, in benchmark_timing
                   timing = timer.timeit(number)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/timeit.py", line 177, in timeit
                   timing = self.inner(it, self.timer)
                 File "<timeit-src>", line 6, in inner
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 599, in <lambda>
                   func = lambda: self.func(*param)
                 File "/home/runner/work/pandas/pandas/asv_bench/benchmarks/arithmetic.py", line 469, in time_apply_index
                   offset.apply_index(self.rng)
                 File "pandas/_libs/tslibs/offsets.pyx", line 87, in pandas._libs.tslibs.offsets.apply_index_wraps.wrapper
                 File "pandas/_libs/tslibs/offsets.pyx", line 2319, in pandas._libs.tslibs.offsets.SemiMonthOffset.apply_index
               AttributeError: 'PeriodIndex' object has no attribute '_addsub_int_array'
               
               For parameters: <SemiMonthBegin: day_of_month=15>
               Traceback (most recent call last):
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 1184, in main_run_server
                   main_run(run_args)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 1058, in main_run
                   result = benchmark.do_run()
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 537, in do_run
                   return self.run(*self._current_params)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 627, in run
                   samples, number = self.benchmark_timing(timer, min_repeat, max_repeat,
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 694, in benchmark_timing
                   timing = timer.timeit(number)
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/timeit.py", line 177, in timeit
                   timing = self.inner(it, self.timer)
                 File "<timeit-src>", line 6, in inner
                 File "/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/asv/benchmark.py", line 599, in <lambda>
                   func = lambda: self.func(*param)
                 File "/home/runner/work/pandas/pandas/asv_bench/benchmarks/arithmetic.py", line 469, in time_apply_index
                   offset.apply_index(self.rng)
                 File "pandas/_libs/tslibs/offsets.pyx", line 87, in pandas._libs.tslibs.offsets.apply_index_wraps.wrapper
                 File "pandas/_libs/tslibs/offsets.pyx", line 2319, in pandas._libs.tslibs.offsets.SemiMonthOffset.apply_index
               AttributeError: 'PeriodIndex' object has no attribute '_addsub_int_array'
```