Add ApplicationContext to the MigrateExternalDiffsWorker

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To make sure we have context for all of the workers (https://gitlab.com/gitlab-com/gl-infra/scalability/issues/131), please add context to the MigrateExternalDiffsWorker which is scheduled from the ScheduleMigrateExternalDiffsWorker through the MergeRequests::MigrateExternalDiffsService.

I couldn't see any of these jobs run in the past 7 days, is this still used?

Some more info in https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/development/sidekiq_style_guide.md#worker-context