This fixes/notates a few deprecations from an audit for the next tech debt cycle.

- ThemeData errorColor and backgroundColor are notated as deprecated in multiple places, but one usage had a different version tag. Updated to match the rest.
- ThemeData.copyWith useMaterial3 was deprecated because it did not do what developers expected - it did not automatically reassign all of the color values and such. (confirmed with @HansMuller) I added a note so that when we later remove it we don't introduce a bug.
- the describeEnum deprecation was tagged wrong, it was a very long PR so by the time it merged the version it was finally tagged with was much later. (https://github.com/flutter/flutter/pull/125016)