Remove GitLab Dedicated support for semantic search until it's available

Changzheng Liurequested to merge
cz-remove-dedicated-support-from-user-doc into master

What does this MR do and why?

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The Semantic Code Search is only compatible with OpenSearch 2+. We are in the process of upgrading OpenSearch to 2.x for Dedicated instances. Before it's done and we can enable the feature, I am removing GitLab Dedicated from supported list.

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