Implement something like secpick for GitLab Pages

Summary

We https://gitlab.com/gitlab-org/release/docs/-/blob/master/general/security/utilities/secpick_script.md for the main repository, but for gitlab-pages the process is completely manual, error prone and complex.

We need tooling which will:

1. determine which pages version are used for the last 3 GitLab releases
2. pick commits to these 3 stable branches on pages