Fix active user count for different number of weeks using same cache (#12025)

Fix #12003

app/presenters/instance_presenter.rb

@@ -21,7 +21,7 @@ class InstancePresenter
   end
 
   def active_user_count(weeks = 4)
-    Rails.cache.fetch('active_user_count';) { Redis.current.pfcount(*(0...weeks).map { |i| "activity:logins:#{i.weeks.ago.utc.to_date.cweek}" }) }
+    Rails.cache.fetch("active_user_count/#{weeks}";) { Redis.current.pfcount(*(0...weeks).map { |i| "activity:logins:#{i.weeks.ago.utc.to_date.cweek}" }) }
   end
 
   def status_count