Move `merge_requests` to gitlab_main_cell

What does this MR do and why?

This MR moves merge_requests to the gitlab_main_cell schema. This is because the records in this table belongs to a project, and projects are cell-local.

✅ - cross-joins arising from this table have been marked using allow_cross_joins_across_databases(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/422085')

4️⃣ foreign keys needs to be converted to Loose foreign key because of this change and it has been marked.

Screenshots or screen recordings

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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 #415204 (closed)