DOC: Remove deprecated use of level for agg functions
- [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

ref #40869, cc @phofl
I assumed these would give warnings when building the docs, and there weren't any. Just did some searching through the userguide and didn't find any. However, there are documented cases of `Series.sub` and `Series.div` using level. I think these are not deprecated (but maybe should be as well?) Sorry I forgot this. I changed them to using groupby when deprecating the keyword

Edit: Yes they are not deprecated > I assumed these would give warnings when building the docs, and there weren't any. Just did some searching through the userguide and didn't find any. However, there are documented cases of `Series.sub` and `Series.div` using level. I think these are not deprecated (but maybe should be as well?)

`.sub` and so on are not deprecated; I think we need the level keyword (as it provides broadcasting behavior) thanks @rhshadrach 