Document warning for KAS in production usage

What does this MR do and why?

Describe in detail what your merge request does and why.

Documents warning that KAS is not ready for production usage.

• gitlab-org/cloud-native/gitlab-operator!300 (merged)
• (internal) https://gitlab.slack.com/archives/C1FCTU4BE/p1637592786349900?thread_ts=1637592627.348500&cid=C1FCTU4BE

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

/review @marcia