BUG: pivot_table mean of integer input casted back to int
- [x] closes #16676
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Remove downcasting and update tests to expect floats according to https://github.com/pandas-dev/pandas/issues/16676#issuecomment-1636827617.

Thanks @austinauyeung! Thanks for the code review @rhshadrach!