Deprecate groupby/pivot observed=False default
- [x] Relates to #17594, Closes #30552
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Had a relatively small 70k data frame that I was trying to do a groupby sum on blow up on me today. This was the reason. I had something like zip codes and cities as categoricals, expected SQL-like groupby but instead got a cartesian product of 'cities' and 'zips'. Sounds like there was some previous desire to explore a new default.

Didn't try to do any wild stuff to keep up with the stacklevel depending on where this was called from.