ERR: Raise error in to_excel when saving datetimes with timezones
- [x] closes #27008
- [x] closes #7056
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Raise a consistent error, regardless of the writing engine, when saving datetimes with timezones in Excel since they are unsupported.