DEPR: Deprecate NDFrame.filter
- [ ] xref #26642
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I propose to deprecate (DataFrame|Series).filter. The method's functionality is already in the ``.loc``method, except the regex functionality, for which I've made a proposal adding regex functionality  to ``.loc`` in #27363.
