BUG: Prevent pd.Series.groupby from showing FutureWarning
- [X] closes #60831
- [ ] [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
- [X] 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.

## Linked Issue
- Future warning observed when series.groupby is called where its name is an integer. The warning seems to stem from `is_in_obj()` defined in `get_grouper()` ; Below indexing results in the warning since `gpr.name` is integer type. For example :

```python
>>>import pandas as pd
>>>arr = [['a', 'a', 'b', 'b'], ['aa', 'bb', 'aa', 'bb']]
>>>idx = pd.MultiIndex.from_arrays(arr, names=['index1', 'index2'])
>>>ser = pd.Series([10, 20, 30, 40], index=idx, name=2)
>>>ser

index1  index2
a       aa        10
        bb        20
b       aa        30
        bb        40
Name: 2, dtype: int64

>>>ser.groupby(ser)
FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]`

```

## Proposed Code Change
- Since `pd.Series` doesn't have columns, added type check so that `is_in_obj()` is called only when `obj` is a `DataFrame`.

```python
    for gpr, level in zip(keys, levels):
        if isinstance(obj, DataFrame) and is_in_obj(gpr):  # df.groupby(df['name'])
            in_axis = True
            exclusions.add(gpr.name)
```

Please let me know if there are any changes needed. Thanks!
The failures here don't look related, but I'm not seeing them fail in other PRs. @mroeschke - have you seen `test_apply_raw_float_frame_lambda` fail in other PRs recently? Thanks @sanggon6107 

Hmm interesting, no I haven't seen that test fail recently. I wonder if it's only happening on the 2.3.x branch for some reason