Add processor to urge engineers to backport code fixes

What does this MR do and why?

This processor will comment on any Merge Request within gitlab-org/gitlab that:

1. Has just been merged AND
2. Has the failureflaky-test label

Example message:

Hi @ddavison , thanks for addressing this flaky test—much appreciated!

Given the impact of flaky tests, we strongly urge you to consider backporting this fix to the last three supported release branches. This helps to ensure stability across those versions and would help prevent recurring test failures and improve CI reliability for ongoing maintenance and patch work.

Please see the GitLab Handbook for information on how to backport changes.

If you need further assistance, feel free to reach out to the #g_delivery Slack channel.

Closes gitlab-com/gl-infra/delivery#21006 (closed)

Expected impact & dry-runs

The expected impact is to drive engagement from engineers that have merged a flaky test fix into master by urging them to also backport this fix.

The direct ping (as opposed to backticked mention) helps ensure accountability and visibility.

References

Implement general improvements to the backport ... (gitlab-com/gl-infra/delivery#21139 - closed)

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

Closes #1653 (closed)