Add bigint index on deployment_merge_requests to fix FK removals

What does this MR do and why?

Add bigint index on deployment_merge_requests to fix FK removals

The primary key from !255297 (merged) was also used as the index for the foreign key and it was removed during the PK swap to bigint. This adds back a temp index to fix the foreign keys. It will be removed when the FK is removed.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #629409 (closed)