Remove @internal from MapperEvent

[gam6itko]

🤔 Why?

this class triggers psalm error

use Cycle\ORM\Entity\Behavior\Attribute\Listen;
use Cycle\ORM\Entity\Behavior\Event\Mapper\Command;
use Cycle\ORM\Entity\Behavior\Event\Mapper\QueueCommand;

final class FooListener
{
    #[Listen(Command\OnCreate::class)]
    #[Listen(Command\OnUpdate::class)]
    public function onEvent(QueueCommand $event)
    {
        assert($event->entity instanceof MySuperBarClass);
        // do things
    }
}

ERROR: InternalProperty
at /home/gam6itko/FooListener.php:x:y
Cycle\ORM\Entity\Behavior\Event\Mapper\QueueCommand::$entity is internal to Cycle but called from FooListener (see https://psalm.dev/176)
        assert($event->entity instanceof MySuperBarClass);