BUG: Timezones are not preserved in Series.mode() output
- [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.

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

---

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

```python
import datetime
times = pd.Series([pd.Timestamp(datetime.datetime.now(), tz='US/Central') for n in range(5)])
times.dtype # datetime64[ns, US/Central] == pd.DatetimeTZDtype(tz='US/Central')
mode_result = times.mode()
mode_result.dtype # dtype('<M8[ns]') == np.dtype('datetime64[ns]')

```

#### Problem description

The Timestamps are normalized to (I think) UTC time, and the original timezone and offset are lost. `Series.median()` does preserve timezones, so I assumed this is just an unhandled edge case. I'm working around this using `scipy.stats.mode`, which does preserve timezones.

#### Expected Output

```python
times.mode().dtype == pd.DatetimeTZDtype(tz='US/Central')
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.7.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           : None.UTF-8
pandas           : 1.2.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.3.4
setuptools       : 49.2.1
Cython           : 0.29.14
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.3
IPython          : 7.24.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 2.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.1
sqlalchemy       : 1.3.23
tables           : None
tabulate         : 0.8.9
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
