Draft: Add one-off migration policy for Category:Jenkins Importer label migration

What does this MR do and why?

This MR re-label and remove Category:Jenkins Importer_[DEPRECATED] with ~"Category:Pipeline Authoring" per #1201 (closed).

Closes #1201 (closed)

Expected impact & dry-runs

• GitLab.org: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/3582127808
  • ✅ 2 epics, aligned with expectations. Epic list
  • ✅ 38 issues, aligned with expectations. Issues list
  • ✅ 20 merge requests, aligned with expectations. Merge requests list

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (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