Dedicated Documentation Updates

What does this MR do and why?

Updates the documentation based on the outcome of https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/2806#note_1548191354

Clarify that we do not support Melbourne region based on https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/2747

Add other regions we do not support based on https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/1405#note_1553826062

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