BUG: Indexing with UTC offset string no longer ignored
- [x] closes #24076
- [x] closes #16785
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Additionally, date strings with UTC offsets that are using in indexing are now validated by the following:

- An error is raised if 2 date strings have different UTC offsets
- An error is raised if the date strings have a UTC offset and the index is not timezone aware.