8 files declare their use of PluginDefinition in Drupal 9

AnnotationBridgeDecoratorTest.php in core/tests/Drupal/Tests/Component/Annotation/Plugin/Discovery/AnnotationBridgeDecoratorTest.php

ContextAwarePluginBaseTest.php in core/tests/Drupal/KernelTests/Core/Plugin/Context/ContextAwarePluginBaseTest.php

ContextAwarePluginTraitTest.php in core/tests/Drupal/KernelTests/Core/Plugin/Context/ContextAwarePluginTraitTest.php

ContextHandlerTest.php in core/tests/Drupal/Tests/Core/Plugin/ContextHandlerTest.php

Contains \Drupal\Tests\Core\Plugin\ContextHandlerTest.

DefaultPluginManagerTest.php in core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php

EntityType.php in core/lib/Drupal/Core/Entity/EntityType.php

LayoutDefinition.php in core/lib/Drupal/Core/Layout/LayoutDefinition.php

SectionStorageDefinition.php in core/modules/layout_builder/src/SectionStorage/SectionStorageDefinition.php