Support gdk-specific labelling on merge requests

What does this MR do and why?

This enables the existing GDK-specific auto-labelling gdk-reliability, gdk-performance and gdk-usability for merge requests. MRs currently have the same checkbox section for the GDK-specific labels like issues, so it would be great if they also had the same automagic labelling.

Expected impact & dry-runs

All MRs in gitlab-org/gitlab-development-kit>.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review