Send master broken incident notifications to DRI group channel

What does this MR do and why?

To keep each group better informed on pipeline status as well as sharing the responsibilities of maintaining test quality, this MR will expand the broken master alerts, which are currently posted in #master-broken channel, to each group channel if a test failure can be traced to a group's feature category.

Posts broken master incidents to group Slack channels based on the incident's group label.

Re: Post broken master pipeline incidents to group'... (gitlab-org/quality/engineering-productivity/team#215 - closed)

OKR: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/2923

Expected impact & dry-runs

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-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

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