COMPAT: match numpy behavior for searchsorted on dt64/td64
- [x] closes #29884
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Should whatsnew be considered a bug or breaking change?
Note the _rebox_native this implements is identical to that in #36171. I guess this needs a whatsnew note for the change in searchsorted? whatsnew added + green