Rollout maintenance policy extension

Context

We are updating the GitLab maintenance policy for bug fixes to the last three releases, aligning the bugfix policy with the security one.

This issue is to take place after the pilot has concluded: https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/21044

Steps to rollout the extension policy

The release tooling change to extend the maintenance policy was implemented as part of #20996 (closed), but it's behind a feature flag. Before we turn on the feature flag, however, we need to manually protect the extended branches.

Adding maintainers to branch rules

We need to manually add branch rules for n-1 and n-2 stable branches to allow maintainers to merge. Look at the details of the n stable branch rule, and copy it to the new branch rules

• https://gitlab.com/gitlab-org/gitlab/-/settings/repository#branch-rules
• https://gitlab.com/gitlab-org/build/CNG/-/settings/repository#branch-rules
• https://gitlab.com/gitlab-org/omnibus-gitlab/-/settings/repository#branch-rules

Turning on the feature flag

• turn on extended_maintenance_policy feature flag: https://ops.gitlab.net/gitlab-org/release/tools/-/feature_flags

Exit Criteria

• Maintenance policy is extended so that the maintainers can merge into the maintained version branches