DEPR: Deprecate DatetimeIndex.offset in favor of DatetimeIndex.freq
- [X] closes #20716
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

> can you run all of the tests and see if any deprecation warnings show (and if they do fix those cases).

none seen in travis/appveyor @jschendel Thanks!