DEPR: remove inplace arg in Categorical methods
- [x] closes #37643
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/v2.0.0.rst` file if fixing a bug or adding a new feature.

I combined these into a single PR given they're all roughly the same deprecation of removing the `inplace` arg from Categorical methods. This can be split into smaller PRs if preferred. 

Deprecations introduced in: #37981, #41118, #41133, #47834
 



thanks @lukemanley  shoot, looks like the whatsnew for 2.0.0 got lost somewhere.  can you do a follow-up to re-push that?  and make sure to update #30228