Invalidate cache with project details when repository is updated

The partial using this cache key was showing stale information due to
the cache not being invalidated.

app/helpers/projects_helper.rb

@@ -171,6 +171,7 @@ module ProjectsHelper
     key = [
       project.route.cache_key,
       project.cache_key,
+      project.last_activity_date,
       controller.controller_name,
       controller.action_name,
       Gitlab::CurrentSettings.cache_key,

changelogs/unreleased/rd-33733-showing-created-date-instead-of-updated-date-in-project-lists.yml

+---
+title: Invalidate cache with project details when repository is updated
+merge_request: 19774
+author:
+type: fixed

spec/helpers/projects_helper_spec.rb

@@ -90,6 +90,10 @@ describe ProjectsHelper do
       expect(helper.project_list_cache_key(project)).to include(project.cache_key)
     end
 
+    it "includes the last activity date" do
+      expect(helper.project_list_cache_key(project)).to include(project.last_activity_date)
+    end
+
     it "includes the controller name" do
       expect(helper.controller).to receive(:controller_name).and_return("testcontroller")