TYP: Series.groupby
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.

similar to #47560

Change `level` typing from `Level` to `IndexLabel`. 

For reference, here are the definitions:
```
IndexLabel = Union[Hashable, Sequence[Hashable]]
Level = Hashable
```


Thanks @lukemanley.