Log Message:
-----------
Mark the ..obvious.. lints as having `state: stable`
This CL changes the `state:` field of the lint documentation records in
pkg/linter/messages.yaml from `experimental` to `stable`.
The CL https://dart-review.googlesource.com/c/sdk/+/479841 was intended
to change the status, and it did remove a paragraph from the
documentation about the lints being experimental, but it was complete in
that it did not change the `state:` fields.