Use new flag to toggle UpdateDefaultContextWorker

What does this MR do and why?

Even though we have the worker to update default contexts on default branch change we are still seeing duplicates on production. This is because the UpdateDefaultContextWorker was using vulnerabilities_across_contexts FF which is not globally enabled. So adding a new FF so that we can toggle this worker as needed independently.

NOTE: We don't see any issues with the worker as of now. So the new FF will enabled on production once the MR is merged and can be toggled back if needed.

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