DEPR: passing timedelta-like to DatetimeIndex constructor, xref #23675
Implements the agreed-upon portions of #23675 so we can keep moving the ball down the field.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
