REF: groupby Series selection with as_index=False
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Currently `.groupby(..., as_index=False)['column']` gives back a DataFrameGroupBy This is because `SeriesGroupBy` doesn't implement `as_index=False`. This gives rise to having a `DataFrameGroupBy` object with `self.obj` being a Series. It also might be a bit surprising to users that this gives them back a DataFrameGroupBy (though this is probably rarely noticed).

Eliminating this case simplifies the states groupby code must consider. It also is a step toward #46944.

There should be no changes in behavior from a user perspective except getting back a SeriesGroupBy and the one noted in the comments below.

cc @jbrockmendel 
Thanks @rhshadrach 