Move KAS image builds to CNG project and build UBI images

Release notes

As of GitLab 14.8, GitLab also builds UBI images. These images are of the form

registry.gitlab.com/gitlab-org/build/cng/gitlab-kas:{$GITLAB_KAS_VERSION}-ubi8

where $GITLAB_KAS_VERSION is the value of https://gitlab.com/gitlab-org/gitlab/-/blob/master/GITLAB_KAS_VERSION at the time of release. For example, with GitLab 14.8, we released the tag v14.8.1-ubi8.

Moreover, in GitLab 14.9, registry.gitlab.com/gitlab-org/build/cng/gitlab-kas will become the default image repository used for GitLab KAS in the Helm chart (see #224 (closed)).

Definition of done

• Research: See what it would take to change kas and agentk to use UBI images as base. This would unblock gitlab-org/cloud-native/gitlab-operator#301 (closed).
• Build KAS image building in CNG: gitlab-org/build/CNG!860 (merged)~
• [-] Update GitLab.com release automation for KAS to use CNG, addressing gitlab-com/gl-infra/delivery#1544 (comment 688046635).
  • Spun out into its own issue, in the %Backlog milestone, #223 (closed)
• [-] Update the GitLab helm chart to refer to the new image
  • Spun out into its own issue but within the same milestone, #224 (closed)
• [-] Move agentk image building to CNG. This should be almost identical to the KAS work, but we focus on KAS first because it unblocks important .com infrastructure work.
  • agentk will continue getting built in this project for now. ARM support is being built in #187 (closed), and UBI support is being tracked in #212 (closed)