DEPR: string indexing along index for datetimes
- [x] closes #31476
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Let's get ourselves one less special case to worry about
> also the ???

placeholder before I had a PR number.  just updated could use some help with the ipython directive errors ping thanks, can you add the entry to the deprecation removal issue as well when you have a sec