BUG: Fix unintentionally added suffix in DataFrame.apply/agg and Series.apply/agg
- [x] closes #36189
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

ci restarted

=========================== short test summary info ===========================
FAILED pandas/tests/plotting/test_groupby.py::TestDataFrameGroupByPlots::test_series_groupby_plotting_nominally_works
= 1 failed, 75150 passed, 1789 skipped, 1180 xfailed, 6 xpassed, 69 warnings in 1028.46s (0:17:08) =
 Thanks @charlesdong1991  @meeseeksdev backport 1.1.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.1.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 752cd42ee5b5a3909c99de1ac35077d1446fce5b
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #36231: BUG: Fix unintentionally added suffix in DataFrame.apply/agg and Series.apply/agg'
```

4. Push to a named branch :

```
git push YOURFORK 1.1.x:auto-backport-of-pr-36231-on-1.1.x
```

5. Create a PR against branch 1.1.x, I would have named this PR:

> "Backport PR #36231 on branch 1.1.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 thank you @simonjayhawkins for pushing this through yesterday! 🙇 

> =========================== short test summary info ===========================
FAILED pandas/tests/plotting/test_groupby.py::TestDataFrameGroupByPlots::test_series_groupby_plotting_nominally_works
= 1 failed, 75150 passed, 1789 skipped, 1180 xfailed, 6 xpassed, 69 warnings in 1028.46s (0:17:08) =

May I ask where was this failure popping up? is it something I should take a look for a potential issue? @simonjayhawkins  not sure. I just add the test failure summary to increase visibility and help track. in this case has occurred b4 https://github.com/pandas-dev/pandas/pull/36147#issuecomment-688883534 failing case is here https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=42259&view=logs&jobId=a3a13ea8-7cf0-5bdb-71bb-6ac8830ae35c&j=4b4d380c-92b5-58cf-5b80-2c6c9b1d5805&t=8eab1181-1335-58ed-7ed5-a271d9d86005&s=d654deb9-056d-50a2-1717-90c08683d50a