Mark background migrations as throttled

We have no particular requirements for the processing of this queue, as
it's meant to be for long-running operations.

app/workers/all_queues.yml

@@ -944,7 +944,7 @@
 - :name: background_migration
   :feature_category: :database
   :has_external_dependencies: 
-  :urgency: :low
+  :urgency: :throttled
   :resource_boundary: :unknown
   :weight: 1
   :idempotent: 

app/workers/background_migration_worker.rb

@@ -4,6 +4,7 @@ class BackgroundMigrationWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
 
   feature_category :database
+  urgency :throttled
 
   # The minimum amount of time between processing two jobs of the same migration
   # class.