Add Ruby 3.0 to CNG image
What does this MR do and why?
https://gitlab.com/gitlab-org/gitlab/-/jobs/6461148652 was failing since Ruby 3.0 is still being tested.
Checklist
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(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Stan Hu