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
> I don't fully remember the previous discussion (should look back at it), but do we want to allow ordered=None in the main constructor?

I had forgotten too but apparently my past self anticipated this and wrote a nice summary here: https://github.com/pandas-dev/pandas/pull/26403#issuecomment-507351745.  Also edited my first comment so this link in included at the top of the PR.  Looks like the decision was to maintain the `ordered=None` behavior.

> If we keep it in the public constructor, we should probably document it?

I can add some documentation for this in a bit (after sleep + work) if we want to advertise this behavior.  There were previously some concerns that this might not be behavior we want to encourage (xref https://github.com/pandas-dev/pandas/pull/26403#discussion_r299160647) since this is in part an implementation detail to handle `"category"`, and the same behavior can be achieved by explicitly specifying `ordered`. @jschendel needs rebase merged master and added a note regarding the usage of `ordered=None` @jschendel Thanks for the update and explanation !