DEPR: SeriesGroupBy._aggregate_named
AFAICT this exists to allow users to apply a different function to each group.  I could be convinced that this is worth supporting, but it shouldn't be shoehorned into `SeriesGroupBy.aggregate` the way it is.

AFAICT this is not documented, supports only one test (parametrized over 4 dtypes):

```
@pytest.mark.parametrize("dtype", ["int64", "int32", "float64", "float32"])
def test_basic(dtype):
    
    data = pd.Series(np.arange(9) // 3, index=np.arange(9), dtype=dtype)

    index = np.arange(9)
    np.random.shuffle(index)
    data = data.reindex(index)

    grouped = data.groupby(lambda x: x // 3)
    [...]
    
    group_constants = {0: 10, 1: 20, 2: 30}
    agged = grouped.agg(lambda x: group_constants[x.name] + x.mean())
    assert agged[1] == 21
```
