Add broken master label nudger automation

What does this MR do and why?

Related to #1174 (closed)

Posts a comment and ping the person who closes a master broken incident if either the root cause label or the flaky test root cause labels are missing.

What needs to be improved?

Right now this processor only reacts to issue.close, meaning if some of us were to add master-brokenflaky-test after the incident is closed, the flaky test reason label will not be prompted. I will iterate on this in a different MR as I think the most important nudge right now is the ~master-broken:: labels

Expected impact & dry-runs

See below comment posted by triage-bot:

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

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