Add rake task for excluding namespaces from free user cap

- to be ran with input file supplied by business

Changelog: added
EE: true

ee/lib/tasks/gitlab/free_user_cap.rake

+# frozen_string_literal: true
+
+namespace :gitlab do
+  namespace :free_user_cap do
+    task exclude_namespaces: :environment do
+      logger = Logger.new($stdout)
+      logger.info('Starting modifying of exclusions for free user cap')
+
+      namespace_file = Settings.source.dirname + 'namespaces_to_exclude.txt'
+
+      if File.file?(namespace_file)
+        File.read(namespace_file).each_line do |namespace_id|
+          namespace_id.chomp!
+          namespace = Namespace.find_by_id(namespace_id)
+          if namespace
+            unless namespace.root?
+              logger.error("Namespace with id of #{namespace_id} is not a top level namespace, skipping.")
+              next
+            end
+
+            if namespace.update(exclude_from_free_user_cap: true)
+              logger.info("Success: namespace with id of #{namespace_id} updated.")
+            else
+              logger.error("Namespace with id of #{namespace_id} failed to update.")
+            end
+          else
+            logger.error("No namespace found with id of #{namespace_id}.")
+          end
+        end
+      else
+        logger.error("No file found. Ensure #{namespace_file} exists with one namespace id per line.")
+      end
+
+      logger.info('Finished modifying of exclusions for free user cap')
+    end
+  end
+end