REGR: NumPy func warning when dropping nuisance in agg, apply, transform

[rhshadrach]

• closes API: GroupBy.agg() numeric_only deprecation with custom function #50538 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

[rhshadrach]

@jorisvandenbossche @jbrockmendel @mroeschke for any thoughts/feedback

[jorisvandenbossche]

In case we keep the current behaviour (no special handling of numeric_only, just passing through), then this is at least a clear improvement in the warning messages!

[rhshadrach]

@jorisvandenbossche - I think trying to change the pass through behavior is too tricky for a patch (this is targeted to go straight into 1.5.3). I do plan to work on it for future versions (e.g. #50624). If you want this to not close #50538, I can remove it from the OP here, but I do think this is the safest route for 1.5.3.

[rhshadrach]

@mroeschke - friendly ping.

[mroeschke]

LGTM. Agreed that improving the exception message here is more reasonable than try to shim numeric_only support of any kind

[mroeschke]

Thanks @rhshadrach