BUG: GH14323 Union of differences from DatetimeIndex incorrect
- [ ] closes #14323 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Sets freq to None when doing a difference operation on a DatetimeIndex
or TimedeltaIndex, rather than retaining the frequency (which can cause
problems with downstream operations). Frequency of PeriodIndex is
retained.
