REGR: DataFrame.update emits spurious warning about downcasting
- [x] closes #57124 (Replace xxxx with the GitHub issue number)
- [ ] [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
- [ ] 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.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This is meant as a stopgap - I plan to finish up #55634 for 3.0 which will avoid any upcasting/downcasting altogether. However I do no like the idea of that PR being put in a patch version since it is an entirely new approach to `DataFrame.update`.

I think this can't go in main because the warning and downcasting behavior has already been removed there.
thx @rhshadrach 

Can you put up a pr with a whatsnew that adds this to main? Only the whatsnew change