Introduce realtime mini-graph job statuses

What does this MR do and why?

Removes FF pipeline_mini_graph_subscription it's been enabled 100% for over a week now 🎉

References

Screenshots or screen recordings

How to set up and validate locally

1. View a pipeline with the mini graph
2. Open a stage dropdown
3. Trigger a job change
4. Open browser dev tools → Network → WS → verify stageJobsUpdated messages for stage job updates
5. Watch the status change realtime
6. Verify closing dropdown unsubscribes from subscription

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 #591935 (closed)