CI: Improve "Regenerate stale policies" job

What does this MR do and why?

This MR tweaks the regenarate-policies CI job:

• Skip MR creation if branch already exists
• Force pushes changes to an existing MR/branch
• Switch to existing remote branch (pending MR
• Only create a new one if it's missing
• Add bot-authored label
• Extract texts into envvars to cleanup code

How to verify on CI

• MR created - !2755 (closed)
• Branch exists, no new changes, push skipped

Manual CI job on MRs

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