Fix duplicate orderedItems in user archive's outbox.json (#31099)

spec/services/backup_service_spec.rb

@@ -60,6 +60,7 @@ RSpec.describe BackupService, type: :service do
 
     aggregate_failures do
       expect(body.scan('@context').count).to eq 1
+      expect(body.scan('orderedItems').count).to eq 1
       expect(json['@context']).to_not be_nil
       expect(json['type']).to eq 'OrderedCollection'
       expect(json['totalItems']).to eq 2