Add release post item label processor

What does this MR do and why?

Adds a new processor to look for open/updated MRs in the www-gitlab-com project. If the MR has the release post item label, it should also have a scoped release post item:: label. If it does not, the bot should comment on the MR asking the assignee(s) (or author if no assignee).

Closes gitlab-com/www-gitlab-com#35299 (closed)

Expected impact & dry-runs

Impact should be minimal as currently only 2 MRs with the release post item label do not have a scoped label: https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/?sort=merged_at_desc&state=opened&label_name[]=release%20post%20item&first_page_size=50

For reference:

1. gitlab-com/www-gitlab-com!74271
2. gitlab-com/www-gitlab-com!135010

But no message should be posted unless the MRs get updated.

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 => current RPM/TA
  • Relevant Slack channels => #release-post https://gitlab.slack.com/archives/C3TRESYPJ/p1760651998144709
  • Engineering week-in-review