The service "update.root" has been changed. In Symfony 6 all services are required to return an object or null. The service "update.root" used to return the root path used to install or update projects, which is a string value and therefore no longer allowed in Symfony 6. The service is only used in testing and there is almost no use in contrib, therefore the decision has been made that the service does not need to be deprecated.
The service has been changed in that it now returns an instance of the class Drupal\update\UpdateRoot. The root path is now returned by calling the method Drupal\update\UpdateRoot::get().
The service "update.root.factory" has been removed. The service was a "hidden" service, therefore the decision has been made that the service does not need to be deprecated.
The 2 modules that were using the "update.root" service will get help with moving to the new situation. This is part of the Symfony 6 in Drupal 10 initiative.
