Replace deprecated STATS_DIRECTORIES with Rails::CodeStatistics.register_directory approach (#33445)

lib/tasks/statistics.rake

@@ -1,21 +0,0 @@
-# frozen_string_literal: true
-
-task stats: 'mastodon:stats'
-
-namespace :mastodon do
-  desc 'Report code statistics (KLOCs, etc)'
-  task :stats do
-    require 'rails/code_statistics'
-    [
-      ['App Libraries', 'app/lib'],
-      %w(Presenters app/presenters),
-      %w(Policies app/policies),
-      %w(Serializers app/serializers),
-      %w(Services app/services),
-      %w(Validators app/validators),
-      %w(Workers app/workers),
-    ].each do |name, dir|
-      STATS_DIRECTORIES << [name, dir]
-    end
-  end
-end