ENH/BUG: consistently cast strs to datetimelike for searchsorted
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

ATM we are pretty inconsistent in how we handle strings for (DTI|TDI|PI|Series).searchsorted.  The underlying EAs will cast scalar strings but not Sequene[str].  The Indexes will raise on both scalars and sequences.  Series follows the EA and casts "2016-01-01" but not `["2016-01-01"]`.

This makes us consistently cast in all cases.