DEPR: Add FutureWarning when matching NaN with NaN in merge/join
- [x] xref #32306 (and a few others, will add them later
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @WillAyd 

I started with raising a FutureWarning when merging nan on nan. This works, if ``left_index`` and ``right_index`` are not both True.

If both are True, we are dispatching to the Index methods. Should we deprecate this for indexes too? Additionally, should we remove this for ``intersection`` of index in the future too? Will continue after receiving feedback :)