ExpireHolds queue worker: DependencySerializationTrait cannot restore private/readonly promoted properties

The ExpireHolds queue worker (src/Plugin/QueueWorker/ExpireHolds.php) implements ContainerFactoryPluginInterface and therefore uses DependencySerializationTrait, but it declares its injected services as promoted private readonly constructor properties.

The PHPStan (Drupal ruleset) job reports four errors: dependencySerializationTraitProperty.unsupportedPrivateProperty and dependencySerializationTraitProperty.unsupportedReadOnlyProperty for each of the two properties.

This is more than lint. DependencySerializationTrait cannot restore private or readonly properties when a queued item is serialised and later resumed, so the worker would come back without its entity type manager and booking manager.

Fix: make both promoted properties protected and drop readonly, the same pattern already used elsewhere in the codebase.