Add requires for YAML and JSON in the DB System Change generator

These were missed in rails/rails@c90a8701

Author: andrewn617

railties/lib/rails/generators/rails/db/system/change/change_generator.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 require "rails/generators/base"
+require "yaml"
+require "json"
 
 module Rails
   module Generators