BUG: pd.to_datetime fixed (#28238)
- [ ] closes #28238
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Fix for KeyError when trying to convert a series with unsorted float index to a DateTime 



@AntonioAndraues
@Vinigl
