Update merging during broken `master` process

Why is this change being made?

With the "Pipelines must succeed" setting now enabled for gitlab-org/gitlab, our previous process of merging during a broken master was no longer possible to follow !77051 (comment 526702420).

This change updates our process to only allow specific urgent merge requests to be merged during a broken master.

This change falls under our Broader Change review process, so needs to be approved by @kwiebers (Backend Engineering Manager of Engineering Productivity).

• Issue: #11080 (closed)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)