Memoize namespace monthly usage to reduce SQL queries

This eliminates an extra subtransaction and an unnecessary SELECT query.
Previously we would call `namespace_usage` twice. Wihle `project_usage`
was memoized, `namespace_usage` was not.

Changelog: performance

ee/app/services/ci/minutes/update_project_and_namespace_usage_service.rb

@@ -58,8 +58,10 @@ module Ci
       end
 
       def namespace_usage
+        strong_memoize(:namespace_usage) do
           ::Ci::Minutes::NamespaceMonthlyUsage.find_or_create_current(namespace_id: @namespace_id)
         end
+      end
 
       def project_usage
         strong_memoize(:project_usage) do