ci: Introduce the 'pipeline:expedite-master-fixing' label

What does this MR do and why?

Corrective action of gitlab-org/quality/engineering-productivity/team#116 (closed).

~"pipeline:expedite-master-fixing" is more generic than ~"pipeline:revert". Note that the label will only activate a fast pipeline if the MR also has master:broken or master:foss-broken.

This is to ensure the label is not over-used for cases that wouldn't fix a broken master (and so that we can audit its usage).

Example of expedited pipeline: https://gitlab.com/gitlab-org/gitlab/-/pipelines/694099215

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.