Bust cache for GL.com billing page

 - adds new cache key and feature flag

ee/app/services/fetch_subscription_plans_service.rb

@@ -42,10 +42,18 @@ class FetchSubscriptionPlansService
   end
 
   def cache_key
-    if @namespace_id.present?
-      "subscription-plan-#{@plan}-#{@namespace_id}"
+    if Feature.enabled?(:subscription_plan_cache_key)
+      if @namespace_id.present?
+        "subscription-plan-#{@plan}-#{@namespace_id}"
+      else
+        "subscription-plan-#{@plan}"
+      end
     else
-      "subscription-plan-#{@plan}"
+      if @namespace_id.present?
+        "subscription-plans-#{@plan}-#{@namespace_id}"
+      else
+        "subscription-plans-#{@plan}"
+      end
     end
   end
 end

ee/config/feature_flags/development/subscription_plan_cache_key.yml

+---
+name: subscription_plan_cache_key
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52174
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/299068
+type: development
+group: group::fulfillment
+default_enabled: false