Standardize datetimelike casting behavior where/setitem/searchsorted/comparison
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Addresses #30699 for eq/ne, but not for inequalities.