Resolve "Background Migrations Visibility Improvement for GitLab engineers - Add resume endpoint"

What does this MR do and why?

This MR was already reviewed/merged, but unfortunately, we had to revert it because the single-db spec job was failing.

This job failed because we had a multiple database test running in a single database environment. I have fixed the problem here.

• Link to the original MR: !95874 (merged)
• Link to the Broken single-db spec issue: #372877 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #346357 (closed)