Problem/Motivation
Running Drupal tests using the core phpunit.xml configuration under PHPUnit 9 yields
PHPUnit 9.5.4 by Sebastian Bergmann and contributors.
Warning: Your XML configuration validates against a deprecated schema.
Suggestion: Migrate your XML configuration using "--migrate-configuration"!
Proposed resolution
Make changes in order to drop deprecated schema elements from the config file. Will it work backwards in PHPUnit 8? If not, either we have to wait dropping PHPUnit 8 support, or find a way to run tests with different config files based on the test runner version.