Once #81417 has been in one or two stable releases we should officially deprecate analysis_options_user.yaml and nudge people to migrate to package:flutter_lints with some kind of a deprecation message.

We need to show a deprecation massage with note to upgrade to people if they

• include analysis_options_user.yaml in their own analysis_options.yaml file (e.g. by specifying include: package:flutter/analysis_options_user.yaml)
• analyze a flutter project that doesn't have a analysis_options.yaml file defined causing the analyzer to implicitly fall back to analysis_options_user.yaml (this fallback is hard-coded in the analyzer, I believe?)

In those situations, the deprecation message must surface for people who run flutter analyze on the command line as well as for people who analyze their code in the IDE (e.g. as a warning in the "Dart Analysis" tab of the IDE).

The deprecation message can link to the migration guide which shows people how to upgrade their project to package:flutter_lints: https://flutter.dev/docs/release/breaking-changes/flutter-lints-package