fix - Use WorkflowInterface instead of Workflow (#36)

This cause an issue with TraceableWorkflow

Author: mpoiriert

src/Admin/Extension/WorkflowExtension.php

@@ -13,7 +13,7 @@
 use Symfony\Component\Workflow\Exception\InvalidArgumentException;
 use Symfony\Component\Workflow\Registry;
 use Symfony\Component\Workflow\Transition;
-use Symfony\Component\Workflow\Workflow;
+use Symfony\Component\Workflow\WorkflowInterface;
 
 /**
  * @author Yann Eugoné <[email protected]>
@@ -107,7 +107,7 @@ public function getAccessMapping(AdminInterface $admin): array
     /**
      * @throws InvalidArgumentException
      */
-    protected function getWorkflow(object $subject, string $workflowName = null): Workflow
+    protected function getWorkflow(object $subject, string $workflowName = null): WorkflowInterface
     {
         return $this->registry->get($subject, $workflowName);
     }