Add migrations for misisng merge_requests indexes for bigint

What does this MR do and why?

Users are reporting issue raised during bigint migration, Swap bigint columns on merge_requests stage one (!203979 - merged)

The root cause would be, some existing indexes from merge_requests table are not present in DB (user DB derived from schema). When we swap bigint indexes, exception raised as those normal indexes do not exist.

In this MR, we add one more migration before the migration for swapping bigint columns, so any missing indexes will be picked up.

References

Issue - Update 18.5.2 to 18.6 : PG::UndefinedTable: ERR... (#581662 - closed)

Screenshots or screen recordings

NA, migration only

How to set up and validate locally

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