Issue labels can now be scoped!

Introduces Scoped Labels, a feature that allows mutually exclusive labels to be applied to issues, merge requests, or epics. This enables custom fields and workflow states, preventing conflicts and cleaning up issue lists. The feature accelerates delivery by allowing teams to define workflow steps and measure the flow of work. It is a paid feature available to Premium and Ultimate tiers for self-managed GitLab or Silver and Gold tiers for GitLab.com.