BUG: Groupby.last accepts min_count but implementation raises
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

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#### Code Sample, a copy-pastable example

```python
df =  DataFrame({"a": [1, 2, 3, 4, 5, 6]})
df.groupby(level=0).last(min_count=1)

```

#### Problem description

This raises 
```
Traceback (most recent call last):
  File "/home/developer/.config/JetBrains/PyCharm2020.2/scratches/scratch_4.py", line 97, in <module>
    print(df.groupby(level=0).last(min_count=1))
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/groupby.py", line 1710, in last
    return self._agg_general(
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/groupby.py", line 1032, in _agg_general
    result = self._cython_agg_general(
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/generic.py", line 1018, in _cython_agg_general
    agg_mgr = self._cython_agg_blocks(
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/generic.py", line 1116, in _cython_agg_blocks
    new_mgr = data.apply(blk_func, ignore_failures=True)
  File "/home/developer/PycharmProjects/pandas/pandas/core/internals/managers.py", line 425, in apply
    applied = b.apply(f, **kwargs)
  File "/home/developer/PycharmProjects/pandas/pandas/core/internals/blocks.py", line 368, in apply
    result = func(self.values, **kwargs)
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/generic.py", line 1067, in blk_func
    result, _ = self.grouper.aggregate(
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/ops.py", line 594, in aggregate
    return self._cython_operation(
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/ops.py", line 547, in _cython_operation
    result = self._aggregate(result, counts, values, codes, func, min_count)
  File "/home/developer/PycharmProjects/pandas/pandas/core/groupby/ops.py", line 608, in _aggregate
    agg_func(result, counts, values, comp_ids, min_count)
  File "pandas/_libs/groupby.pyx", line 906, in pandas._libs.groupby.group_last
AssertionError: 'min_count' only used in add and prod

Process finished with exit code 1
```

#### Expected Output

I would expect, that this function does not accept ``min_count`` as input (similar to ``Resample.last``) and raises the same error as described in #37768 
Additonally the docs need adjustments, because ``min_count`` is documented. This probably holds true for most of the functions mentioned in https://github.com/pandas-dev/pandas/issues/37768#issuecomment-727009192 too.

#### Output of ``pd.show_versions()``

<details>

master

</details>
