Trigger workItemUpdated subscription when related MR is updated

Mario Celirequested to merge
479408-wi-real-time-update-on-mr-update into master

What does this MR do and why?

We have associations between MRs and issues through the merge_requests_closing_issues table, and we display some information on the work item detail view. If the MR is changed, we need those changes to be reflected on the work item view.

MR acceptance checklist

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

How to set up and validate locally

It's hard to test GQL subscriptions locally and I'm not aware of a good way to do it yet

Related to #479408 (closed)

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