BUG: Resolving fallback from skipna flag in groupby().sum()
- [x] closes #20824 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Issue #20824 found that a fallback was occuring when df.Groupby().sum() was called with the skipna flag. This was occurring because the _cython_agg_general function was not accepting the argument, which has now been fixed. The fallback still occurs with strings in the df, however this is a deeper issue stemming from the _aggregate() call in groupby/ops.py (line 572).