REGR: fillna on datetime64[ns, UTC] column hits RecursionError
- [ ] closes #38851
- [ ] 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

@jbrockmendel this reverts #37040 in it's entirety for backport

feel free to push to this branch if we **need** any of the ancillary cleanup on 1.2.x otherwise can open a new PR to add those cleanups back to master.