Add section::growth issue default workflow labeller policy

What does this MR do and why?

To follow up on the work we previously did for groupgeo, sectiongrowth are now ready to adopt this "mandatory workflow label" policy 💪

See gitlab-org/developer-relations/contributor-success/team-task#422

Expected impact & dry-runs

https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/6611764707

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