DEPR: min_periods=None behavior for Rolling.count
- [x] closes #31302
- [x] closes #35579 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Additionally refactors `count` to take the `_apply` path to make this op consistent with the others.