Reduce N+1 queries from Projects::MergeRequestsController#index action related to protected branches and code owners

After resolving #276944 (closed), there are still N+1 queries like this when viewing https://gitlab.com/gitlab-org/gitlab/-/merge_requests:

SELECT "protected_branches".* FROM "protected_branches" WHERE "protected_branches"."project_id" = 278964 AND "protected_branches"."code_owner_approval_required" = TRUE

This can be seen from pb when viewing the specified page. The goal of this issue is to reduce the number of queries executed to further improve the action.