DEPR: groupby numeric_only default
- [x] closes #46072 (Replace xxxx with the Github issue number)
- [x] [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).
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

There are two cases we want to emit a deprecation warning for DataFrameGroupBy:
 - numeric_only is not specified and columns get dropped. In this case emit a warning that the default of numeric_only will change to False in the future.
 - numeric_only is specified to False and columns still get dropped. In this case emit a warning that the op will raise in the future.
Thanks @jreback; filterwarnings has been removed and the helper had been added. very nice @rhshadrach  I think this causes the doc build to fail with multiple of these warnings:

> <ipython-input-97-5805a339bf74>:1: FutureWarning: The default value of numeric_only in DataFrameGroupBy.sum is deprecated. In a future version, numeric_only will default to False. Either specify numeric_only or select only columns which should be valid for the function.
>   (bb.groupby(['year', 'team']).sum()

Would probably need to update the documentation to avoid these FutureWarnings. Thanks @twoertwein - will do a follow up.