DEPR: deprecate default of skipna=False in infer_dtype
- [x] follows up on the changed in #17066
- [x] tests adapted / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I know that `v.0.22` is generally not counted towards the "3 major releases between warning and deprecation" policy, but since this is a private method, I'm thinking it should be OK?

Would make life easier in a couple of places (especially in cleaning up some of the casting code).