BUG: #35830 Restore index & slice by date to DatetimeIndex
#35830 DatetimeIndex has supported indexing by date for many many years. This was broken after 1.0.5. This attempts to restore support by converting a date to Timestamp in `get_loc` and `_validate_searchsorted_value`. This restores indexing slicing, and  `asof` but  `in` does not work. It seems the best solution would be to do the conversion in `DatetimeLikeArrayMixin._validate_scalar` but this breaks a test looking for an expected `FutureWarning` in `TestDataFrameIndexing::test_loc_setitem_datetime_coercion` warning a `Timestamp` can't be compared to a `date`

- [x] closes #35830
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry
