Removal of nonfunctional `package_api_docs` lint

[parlough]

Background

The lint hasn't been functional for a long time. However, it has also been around and added to analysis options for a long time. Instead of trying to fix it and introducing a bunch of new warnings on SDK update, let's deprecate and promptly remove it.

Keeping it around in its current state sets false expectations for developers who expect it to be ensuring their package's libraries are well documented.

Replacement

If anyone is interested in proposing implementing a replacement, it can be added under a new name, so developers can be aware when they opt-in to enabling its new behavior. Please open a new lint proposal if you are interested in that work.

Removal plan

1. Publish this issue as notice and for pointing to as a reference.
2. Remove it from analysis_options.yaml files across the dart-lang and flutter organizations.
3. Deprecate the lint in the SDK before the first Dart 3.7 beta release (cl/389585).
4. Remove usages of the lint elsewhere across the open-source ecosystem.
5. Depending on how the deprecation goes, remove the lint in either Dart 3.7 or 3.8.