Bug: to_csv coercing datetime columns to same format
- [x] closes #21734
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

@phofl the web & docs build failed but hard to tell why, can you rebase The web and docs failure was unrelated. An inventory was not available. greenish