Services ids from the ContainerBuilder can be integers

`ContainerBuilder::getServiceIds()` can return integers. It tries to cast all ids to string on input but because they're used as array keys, they are converted to integer if they are numeric

related issue : https://github.com/sonata-project/SonataAdminBundle/pull/5638

It was easier to to a PR against sonata-admin-bundle but I think it should be fixed in Symfony too