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

This PR deprecates the ``valid`` method on ``pd.Series``. This method duplicates ``dropna`` and should be deprecated according to #18262.