DEPR: Change default value for CategoricalDtype.ordered from None to False
- [X] closes #27203
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

A quick summary of the deprecation details can be found here: https://github.com/pandas-dev/pandas/pull/26403#issuecomment-507351745