Move milestone permissions to milestone docs and Remove project joins

What does this MR do and why?

We are writing a migration to move milestone permissions to milestone docs and also Remove project joins from the search query.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415221 (closed)