Remove unclear "Proof of Concept" wording

What does this MR do?

The "proof of concept" wording raised some concerns about Helm deployments themselves being officially supported or not. We use the terms "Beta", "Experimental" and "General Availability" to talk about various levels of official support, so the undefined term here can cause confusion.

Related issues

Slack conversation for context:

Hi team, would we support our customers around features that are in a PoC phase (e.g. GitLab Helm Chart or GitLab Operator)? Could they open a ticket if they face any issues?

Thanks a lot! So if it's not stated as Beta / Experiment in the documentation, it can be considered as GA? E.g. GitLab Operator is " is under active development and is not yet suitable for production use". Would that be considered as GA?

I think the “Proof of Concept” wording for Helm Charts and Operator only applies to the configuration examples we provide, not to “using GitLab with this technology”.

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