Mark the StageUpdateWorker as idempotent

This worker was already idempotent, it updates the status of a stage
based on the jobs inside it.

This also means we can safely deduplicate it.

app/workers/all_queues.yml

@@ -849,7 +849,7 @@
   :urgency: :high
   :resource_boundary: :unknown
   :weight: 5
-  :idempotent: 
+  :idempotent: true
 - :name: pipeline_processing:update_head_pipeline_for_merge_request
   :feature_category: :continuous_integration
   :has_external_dependencies: 

app/workers/stage_update_worker.rb

 # frozen_string_literal: true
 
-class StageUpdateWorker # rubocop:disable Scalability/IdempotentWorker
+class StageUpdateWorker
   include ApplicationWorker
   include PipelineQueue
 
   queue_namespace :pipeline_processing
   urgency :high
 
+  idempotent!
+
   def perform(stage_id)
     Ci::Stage.find_by_id(stage_id)&.update_legacy_status
   end

spec/workers/stage_update_worker_spec.rb

@@ -12,6 +12,15 @@ describe StageUpdateWorker do
 
         described_class.new.perform(stage.id)
       end
+
+      it_behaves_like 'an idempotent worker' do
+        let(:job_args) { [stage.id] }
+
+        it 'results in the stage getting the skipped status' do
+          expect { subject }.to change { stage.reload.status }.from('pending').to('skipped')
+          expect { subject }.not_to change { stage.reload.status }
+        end
+      end
     end
 
     context 'when stage does not exist' do