Add information about reproducing GitLab Dedicated issues in UAT instance

Why is this change being made?

It's important being able to replicate things in the same environment that our customers are using. Following up on a Slack discussion:

Q: It would have been incredibly useful for me to have a Dedicated test instance, where I could have reproduced the change myself and observed what I see in OpenSearch after doing so. Currently it’s very hard for me to tell what part of the observed oddities is due to differences between Dedicated and my test instance. Is it possible to provide Support with a Dedicated instance that works just like a Prod Dedicated instance for reproduction purposes like that?

A: Re point 3, this is a great idea. We could give you access to our longliveduat instance perhaps? Related issue: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/968 … I think as long as there is visibility with the team, this would be reasonable. For example, perhaps, just announce in #g_dedicated-team if you’re looking to make a change, and follow up with another message once it’s been reverted. Obviously, it’s critical for the support teams to be able to understand what’s happening in Dedicated environments, and to me, using UAT seems like a good option. Perhaps we should add some documentation to this affect as a proposal.

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Marked as Draft because the credentials are not actually in the Vault yet – needs corresponding issue in GitLab Dedicated Issue Tracker.

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