Deprecate support for OpenShift 4.12

Summary

OCP 4.12 will be EOL on 2024-07-17: https://docs.openshift.com/dedicated/osd_architecture/osd_policy/osd-life-cycle.html#sd-life-cycle-dates_osd-life-cycle

Acceptance Criteria

• OCP 4.12 jobs are removed
• OCP 4.12 is moved to Deprecated in our install documentation: https://docs.gitlab.com/operator/installation.html?tab=OpenShift#cluster
• OCP 4.12 cluster is decommissioned (this needs backports of the CI job removal)