DEPR: Deprecate ordered=None for CategoricalDtype
- [X] closes #26336
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

All of the logic related to breaking changes looks to be consolidated in `CategoricalDtype.update_dtype`, so I'm raising the warning there when a breaking change would occur and letting it flow through from there.

Aside from `CategoricalDtype.update_dtype`, I think the only other place where a breaking change can manifest is in `astype` as described in the issue.  Will need to double check the test logs to see if anything else triggered a warning though.

cc @TomAugspurger @jorisvandenbossche 
