Zoekt support on GitLab Dedicated instance

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In preparing the GA of Exact code search backed by Zoekt, we need to consider the deployment scenario for Dedicated instances. This issue is a place holder for related tasks and it can be broken down later.

Potential work

• ✅ We need to remove the FFs for the logic that's required by GA. This is a list of Zoekt FFs we have.
• 🏗️ We need to provide admin setting(s) to switch between Zoekt code search and Elasticsearch based code search. Eventually, we will deprecate and remove the latter. But there's a period of time we allow to switch between them gitlab-org#18677 (closed)
• ✅ We need to provide the sizing guidelines for different scales of the instances.
• 🏗️ We may also need to provide cost estimation for using Zoekt code search. Note: I believe that we have a pretty good idea already #535254
• Add Geo support (might be not needed for the initial launch similar to Advanced Search. Pending decision)