Add policy for Seeking community contributions

What does this MR do and why?

This adds a policy with multiple rules for removing the Seeking community contributions label from issues and MRs. This borrows heavily from label-accepting-merge-requests.yml, because the labels are similar.

This is part of the epic to deprecate Accepting merge requests.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

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