Don't assume group label prefix

What does this MR do and why?

We don't assume any particular format for group/stage/section labels in DevopsLabels, we just use the values from https://about.gitlab.com/groups.json / https://about.gitlab.com/stages.json / https://about.gitlab.com/sections.json. This allows to define "unlisted" groups such as Engineering Productivity, GDK, Infrastructure etc. instead of having the concept of "department" in the DevopsLabels.

This will solve https://new-sentry.gitlab.net/organizations/gitlab/issues/574826/?project=20&query=is%3Aunresolved&referrer=issue-stream&stream_index=4.

Close #1461 (closed).

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