6 files declare their use of MessageQueueItem in Courier 8

CourierMaintenanceFormTest.php in src/Tests/CourierMaintenanceFormTest.php

CourierManager.php in src/Service/CourierManager.php

CourierManagerTest.php in tests/src/Kernel/CourierManagerTest.php

CourierMessageQueueManagerTest.php in tests/src/Kernel/CourierMessageQueueManagerTest.php

CourierTest.php in src/Tests/CourierTest.php

Contains \Drupal\courier\Tests\CourierTest.

MessageWorker.php in src/Plugin/QueueWorker/MessageWorker.php

Contains \Drupal\courier\Plugin\QueueWorker\MessageWorker.