Fix issues assignees

With this MR we will always assign issues to the active release managers.

In order to make so, I made the first step for fixing gitlab-com/gl-infra/delivery#810 (closed)

In order to avoid the aforementioned issues, we now authorize release manager from a list of eligible users

The release manager gitlab group is now designed to be kept in sync with the active release manager for the milestone.

This change broke ReleaseManagers::Schedule#authorized_release_managers(version) as it used to validate users on the @gitlab-org/release/managers gitlab group.

This merge request will remove such check, replacing it with a validation against config/realease_managers.yml.