Restrict use of CTE fence to dashboards/projects#index

In https://gitlab.com/gitlab-org/gitlab/merge_requests/23754, we added a
CTE optimization fence to ensure loading a user's projects is always
fast.

However, using a CTE for the dashboard/projects/starred endpoint is less
efficient than the default query because that endpoint also loads public
and internal projects. For now, we can just restrict use of this CTE for
the `index` action.

In the future, we may want to put this logic inside ProjectsFinder.

Closes https://gitlab.com/gitlab-org/gitlab/issues/201994

app/controllers/dashboard/projects_controller.rb

@@ -66,7 +66,7 @@ class Dashboard::ProjectsController < Dashboard::ApplicationController
     @total_user_projects_count = ProjectsFinder.new(params: { non_public: true }, current_user: current_user).execute
     @total_starred_projects_count = ProjectsFinder.new(params: { starred: true }, current_user: current_user).execute
 
-    finder_params[:use_cte] = Feature.enabled?(:use_cte_for_projects_finder, default_enabled: true)
+    finder_params[:use_cte] = true if use_cte_for_finder?
 
     projects = ProjectsFinder
                 .new(params: finder_params, current_user: current_user)
@@ -79,6 +79,11 @@ class Dashboard::ProjectsController < Dashboard::ApplicationController
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
+  def use_cte_for_finder?
+    # The starred action loads public projects, which causes the CTE to be less efficient
+    action_name == 'index' && Feature.enabled?(:use_cte_for_projects_finder, default_enabled: true)
+  end
+
   def load_events
     projects = load_projects(params.merge(non_public: true))