Update GITLAB_KAS_VERSION to 15.0.0

What does this MR do and why?

Highlights:

• Removed Cilium alerts (already announced, see https://docs.gitlab.com/ee/update/removals.html#container-network-and-host-security)
• Fixed support for using client certificates for Redis connections

For a full list of changes, see gitlab-org/cluster-integration/gitlab-agent@v14.10.0...v15.0.0

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