BUG/API: SeriesGroupBy reduction with numeric_only=True
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

This addresses the SeriesGroupBy (easier) half of #41291:

1) When we have numeric_only=True and non-numeric data in a SeriesGroupBy reduction, raise NotImplementedError (matching Series behavior) instead of falling back to an often-wrong fallback (xref #41341)
2) When the user doesn't explicitly pass `numeric_only=True`, change the default to False for SeriesGroupBy, leaving DataFrameGroupBy unaffected.