DEPR: Deprecate use of strings denoting units with 'M', 'Y' or 'y' in pd.to_timedelta (36666)
- [x] closes #36666
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
