DEPR: astype dt64<->dt64tz
- [x] closes #38622
- [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
- [x] whatsnew entry

> tested with DTI, Series, DTA? (i think so, just confirming). 

Yes

> do we allow this with Timestamp? (obviously deprecate if we do)

No, there is no Timestamp.astype great