Gitaly: Promote Gitaly on K8s to Beta

What does this MR do and why?

Gitaly on K8s is ready for Beta support. These docs have been published for a year now, and we feel confident in K8s as a valid deployment for Gitaly.

This MR promotes the feature to Beta whilst adding it to the Early Access Program.

References

This proposal includes more context for why we want to release Gitaly K8s in beta.

MR acceptance checklist

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