Clean up the post deployment migration generator (#24233)

lib/generators/post_deployment_migration/templates/migration.erb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
+  disable_ddl_transaction!
+
+  def change
+  end
+end