For phpBB the migration folder and objects are called migration. For extensions you have to use migrations (plural). This is very confusing.
Secondly, the extension migration also needs to extend \phpbb\db\migration\migration

So an extension migration file currently has a namespace with the plural form "migrations" and extends singular "migration".

namespace derky\sortablescaptcha\migrations;

class mymigration extends \phpbb\db\migration\migration