BUG: to_datetime(tz_mix, utc=True) converts to UTC
- [x] closes #48678 (Replace xxxx with the Github issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/v1.5.1.rst` file if fixing a bug or adding a new feature.

Can merge on greenish I think thx @mroeschke 