DEPR: convert_datetime64 parameter in to_records()
- [x] closes #18160
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

As noted in the original issue, the underlying NumPy bug seems to be fixed so this parameter may no longer be needed. 

I also changed the default value for ``convert_datetime64`` from ``True`` to ``False`` and only give a ``FutureWarning`` when ``convert_datetime64=True`` is passed but I'm not sure if this is more appropriate than leaving ``True`` as the default and always giving a ``FutureWarning``.