DEPR: Index.is_monotonic for Index.is_monotonic_increasing
- [x] closes #21335
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

closes #21335 i think thanks @mroeschke 