Stop rebuilding old OS images

What does this MR do?

Implements our image support policy from https://gitlab.com/gitlab-org/ci-cd/shared-runners/macos#macos-and-xcode-version-support.

• Images older than Big Sur are frozen, meaning we don't change anything besides the password (done by the release job on main). The policy mentions being able to do security updates and runner upgrades, but this is not implemented yet.
• The Big Sur image is in maintenance, meaning the toolchain is frozen, but we can update Xcode (done by the xcode 12 job).

In another MR, I'll add the next image mentioned in the policy to re-enable the full toolchain build, but this MR should at least allow us to release images again.

Why was this MR needed?

main is broken

What's the best way to test this MR?

What are the relevant issue numbers?