Prepare foreign keys for bigint conversion on merge_requests Stage One

What does this MR do and why?

Stage One, Step Two, for merge_requests table bigint migration, as part of swapping merge_requests table int columns.

The whole columns are, COLUMNS = %i[latest_merge_request_diff_id assignee_id author_id id last_edited_by_id merge_user_id milestone_id source_project_id target_project_id updated_by_id].freeze

We focus on columns assignee_id, merge_user_id, updated_by_id, milestone_id, source_project_id in stage one with this MR.

• Ensure int to bigint backfill job is finished
• Synchronously create indexes for non-gitlab.com envs
• Create not valid foreign keys
• Queue asynchronous foreign key validation

References

Issue - Swap MergeRequest ID columns (Step 2) (#507695)

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Before	After

How to set up and validate locally

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