Fix GitHub breaking API change of `maintainers` in `Organization.create_team` (#3291)

Fixes #3278 by aligning with GitHub's breaking API change.

Key Changes:

* Change `maintainers` type to `str`, from `int.
* ~~Introduce targeted test for this change~~
* Update existing Organizations unit test and content to account for
`str` change

Co-authored-by: Enrico Minack <github@enrico.minack.dev>