DEPR: group by one element list gets scalar keys
- [x] closes #58858

changed the behavior: group `keys` to be `1-tuples` instead of `scalar` when grouping with a `length-1 list`
I deprecated the behavior for `groups`: group `keys` are now 1-tuples instead of scalars when grouping with a length-1 list.
I fixed the tests except for this one `test_groupby_raises_category_on_category` in `pandas/tests/groupby/test_raises.py`. I got errors like:
```
FAILED pandas/tests/groupby/test_raises.py::test_groupby_raises_category_on_category[by3-False-idxmin-False-agg] - ValueError: Can't get idxmin of an empty group due to unobserved categories. Specify observed=True in groupby instead.
```
@rhshadrach, could you please advise me on how to fix this test? > @rhshadrach, could you please advise me on how to fix this test?

Sorry for the delay! I believe you've resolved this, let me know if not. > Sorry for the delay! I believe you've resolved this, let me know if not.

Thanks @rhshadrach for reviewing this PR. I moved the FutureWarning from Grouping to GroupBy.groups as you suggested and added a note to v3.0.0. Could you please take a look at my changes? Thanks @natmokval!