Remind about guidelines only for main branch merge requests

What does this MR do and why?

When I was working on cross-project merge requests and conflict resolution I noticed that the reactive triage is reminding default branch MR guidelines for MR targeting feature branches (where those rules do not apply).

Whit this merge request, we limit the processor to MR targeting main and master branches.

Expected impact & dry-runs

Comments like gitlab-org/gitlab!125576 (comment 1461807346) should no longer exist

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