Prepare index async for merge requests on author and target project id
What does this MR do and why?
In this MR we are preparing index async so it can be then used in !99966 (merged).
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #375284 (closed)