Issue #3252257 by Spokje, beatrizrodrigues, longwave, paulocs, mondrake: Remove PHPUnit 8 support

composer/Plugin/VendorHardening/Config.php

core/lib/Drupal/Core/Composer/Composer.php

core/lib/Drupal/Core/Test/TestDiscovery.php

core/scripts/run-tests.sh

...sts/Drupal/TestTools/PhpUnitCompatibility/ClassWriter.php

.../PhpUnitCompatibility/PhpUnit8/TestCompatibilityTrait.php

core/tests/Drupal/Tests/ComposerIntegrationTest.php

...tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php

core/tests/Drupal/Tests/Listeners/HtmlOutputPrinter.php

core/tests/Drupal/Tests/PhpUnitWarningsTest.php

core/tests/bootstrap.php