Deprecate level keyword for dataframe and series aggregations
- [x] closes #39983
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

This would deprecate the level keyword if we want to move forward with this
> wouldn't be averse to change some of the tests to use the groupby instead of the reduction method to remove / avoid the warnings (some, not all)

Most of the tests are pretty basic and already covered in groupby, the more complicated ones are mostly checking if the aggregation with level equals a groupby, so don't know if this makes sense here? > > wouldn't be averse to change some of the tests to use the groupby instead of the reduction method to remove / avoid the warnings (some, not all)
> 
> Most of the tests are pretty basic and already covered in groupby, the more complicated ones are mostly checking if the aggregation with level equals a groupby, so don't know if this makes sense here?

kk sounds good thanks @phofl 