BUG: `tseries.offsets.MonthEnd` is leaving out full 24h of period
#### Code Sample, a copy-pastable example

```python
>>> pd.Timestamp('2021-05-20') + pd.tseries.offsets.MonthEnd
Timestamp('2021-05-31 00:00:00')
```

`MonthEnd` gets me the first second of the last day, while I was expecting the last second of the last day of month (`Timestamp('2021-05-31 23:59:59.99999999')`). This yields strange consequences as showed below.

#### A consequence of unexpected behavior

When using `df.resample('M').ffill()` to get the last known value for a month, data for the last 24h of a month are ignored.

```python
>>> df=pd.DataFrame({'val': {
  pd.Timestamp('2021-03-24 12:03:00'): 1.0,
  pd.Timestamp('2021-03-31 12:00:00'): 1001.0,
  pd.Timestamp('2021-04-28 12:03:00'): 1.0,
  pd.Timestamp('2021-04-30 23:59:00'): 1002.0,
  pd.Timestamp('2021-05-30 00:00:01'): 1.0,
  pd.Timestamp('2021-05-31 12:00:00'): 1003.0
}})

>>> df
                        val
2021-03-24 12:03:00     1.0
2021-03-31 12:00:00  1001.0
2021-04-28 12:03:00     1.0
2021-04-30 23:59:00  1002.0
2021-05-30 00:00:01     1.0
2021-05-31 12:00:00  1003.0

>>> df.resample('M').ffill()
            val
2021-03-31  1.0
2021-04-30  1.0
2021-05-31  1.0
```

The expected behavior for the `ffill` operation is >1000 for all `MonthEnd`.

On the other hand, a `sum` operation works as expected:

```python
>>> df.resample('M').sum()
               val
2021-03-31  1002.0
2021-04-30  1003.0
2021-05-31  1004.0
```

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

<details>

```
INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.5.0
Version          : Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.20.1
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : None
pymysql          : 1.0.2
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.20.0
pandas_datareader: 0.9.0
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.1
sqlalchemy       : 1.3.23
tables           : None
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : None
numba            : 0.53.1
```

</details>
