DEPR: Timestamp.freq
- [x] closes #15146
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The motivation here is that I've concluded that DatetimeArray.freq _must_ be removed, so for internal consistency Timestamp.freq should be removed too.

The main user-facing behavior this will affect is Timestamp.is_month_start, is_month_end, is_quarter_start, is_quarter_end, is_year_start, is_year_end, for which I guess we'll offer users an alternative(?)