DOC: json_normalize breaking changes in pandas 3.0.0
For context: #51542 & #57422.

As discussed in the above issues,` json_normalize` has breaking changes in 3.0.0.  This is a quick fix to make that more salient in the update documentation, as I very much expect there to be code in the wild that uses the current json_normalize behaviour.

- [x] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature.

pre-commit.ci autofix Thanks @JMBurley 