Draft: Add one-off migration policy for Category:Pipeline Authoring label migration

What does this MR do and why?

This MR re-labels ~"Category:Pipeline Authoring" to Category:Pipeline Composition, and assigns Category:Pipeline Composition to grouppipeline authoring per #1241 (closed).

Closes #1241 (closed)

Expected impact & dry-runs

• Relabel ~"Category:Pipeline Authoring" to Category:Pipeline Composition in GitLab.org: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/3858563303
  • ✅ 44 epics, aligned with expectations. Epic list
  • ✅ 3231 issues, aligned with expectations. Issues list
  • ✅ 777 merge requests, aligned with expectations. Merge requests list
• Assign Category:Pipeline Composition to grouppipeline authoring in GitLab.org: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/3858563300
  • ✅ 0 epics, aligned with expectations.
  • ✅ 496 issues, aligned with expectations. Issues list
  • ✅ 56 merge requests, aligned with expectations. Merge requests list

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