Put the rename uploader column into a change method

gems/houdini_upgrade/lib/generators/cw_to_activestorage/templates/migrate/rename_uploader_columns.rb

@@ -1,5 +1,6 @@
 # License: AGPL-3.0-or-later WITH Web-Template-Output-Additional-Permission-3.0-or-later
 class RenameUploaderColumns < ActiveRecord::Migration[5.2]
+    def change
         require "houdini_upgrade"
         HoudiniUpgrade::UPLOADERS_TO_MIGRATE.each do |table|
             table.fields.each do |field|
@@ -7,3 +8,4 @@ class RenameUploaderColumns < ActiveRecord::Migration[5.2]
             end
         end
     end
+end