DEPR: apply/agg using agg/apply
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

A possible step toward #35725. Internally, agg may use apply and apply may use agg. Straightening these out so that agg always uses agg and apply always uses apply before changing the behavior of agg (without impacting apply).

This PR emits FutureWarning but with (a) no alternative to not get the warning and (b) no clear way for users to know what the future result may be (it may not even change). The only alternative ways forward I see here are

 a. Change behavior with no warning
 b. An option to switch between old/new implementation of apply/agg/transform as mentioned in #41112