DEPR: casting date to dt64 in maybe_promote
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] 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

Once enforced, maybe_promote behavior will match DatetimeArray._validate_setitem_value and infer_dtype_from_scalar

sits on top of #39743