DEPR: Deprecate DataFrame.last and Series.last
- [X] closes #53692 
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] 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/vX.X.X.rst` file if fixing a bug or adding a new feature.

@MarcoGorelli Is something going on with the CI environment? The tests that are failing for this PR are not failing locally for me when I run pytest. 

The same tests also started failing in #53439 this morning after I updated my entry in whatsnew v2.1.0. None of the failing tests seem to have anything to do with whats being changed in the PR hmmmm @MarcoGorelli The Doc build test failure seems to be unrelated

The docstring test is failing because the warning message that I imeplemented is executing. Since this method is being deprecated what would be the recommended solution in this instance? 

Should I add an additional note in the docstring to  create a mask and filter using `.loc` instead of `last` and then add code that shows how to do that? I don't believe this was handled in the PR for deprecating [series.first and DataFrame.first](https://github.com/pandas-dev/pandas/pull/53419/files#diff-1a2e3df0db7dd8bddc2ec4bff9de8a7a55e328e6c32e2cecde761dc9549fcd46)
 thanks @rmhowe425 - does it work to `doctest: skip` the failing doctest? @MarcoGorelli Doc tests passing now. Ready for review. :smile: 

The two failing unit tests still don't seem to be related to the PR.

Just out of curiosity, when updating doc strings for methods that will be deprecated, is the correct approach to skip doc tests for those methods?