Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
This uses https://gitlab.com/gitlab-org/gitlab-styles which includes a shared RuboCop config and custom cops that can be reused by multiple projects.
Will allow to solve #37711.
We can still define custom cops as before, and regularly migrate them to the gem if this is more practical (or for testing/debugging purpose).
So that we can avoid duplicating our RuboCop config and custop cops.