DEPR: deprecate .add_prefix and .add_suffix
- [x ] xref #18262
- [ x] tests added / passed
- [ x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ x] whatsnew entry

Deprecate ``.add_prefix`` and ``.add_suffix`` as per #18262.

An issue is that ``.add_prefix`` and ``.add_suffix`` call the like-named methods in ``internals.py::BlockManager``. Should ``BlockManager.add_prefix/add_suffix`` be changed somehow, e.g. have a warning as well?

EDIT: This is my first deprecation PR, so I'll be more than appreciative of criticism, so I understand these better in the future.

EDIT 2: With ``.add_prefix``/``add_postfix`` deprecated, I think it would be good to show in the doc strings for ``.rename`` how to do similar things, so I've changed its doc string a bit.