Fix Style/OpenStructUse offenses in group cluster controller specs

Changelog: other

.rubocop_todo/style/open_struct_use.yml

@@ -12,7 +12,6 @@ Style/OpenStructUse:
   - lib/gitlab/import_export/after_export_strategies/base_after_export_strategy.rb
   - lib/gitlab/testing/request_inspector_middleware.rb
   - lib/mattermost/session.rb
-  - spec/controllers/groups/clusters_controller_spec.rb
   - spec/controllers/projects/clusters_controller_spec.rb
   - spec/factories/go_module_versions.rb
   - spec/factories/wiki_pages.rb

spec/controllers/groups/clusters_controller_spec.rb

@@ -309,7 +309,8 @@ RSpec.describe Groups::ClustersController do
           .to receive(:expires_at_in_session).and_return(1.hour.since.to_i.to_s)
         allow_any_instance_of(GoogleApi::CloudPlatform::Client)
           .to receive(:projects_zones_clusters_create) do
-          OpenStruct.new(
+          double(
+            'instance',
             self_link: 'projects/gcp-project-12345/zones/us-central1-a/operations/ope-123',
             status: 'RUNNING'
           )