API/DEPR: Change default skipna behaviour + deprecate numeric_only in Categorical.min and max
- closes #25303 and follow up https://github.com/pandas-dev/pandas/pull/27801#discussion_r313075771
- 1 test added
- passes `black pandas`
- passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- whatsnew entry
