Merge branch '6.0' into 6.1

* 6.0:
  [Security] fix compat with older session payloads
  Fix transient test
  Fix transient test

Author: nicolas-grekas

Authentication/Token/AbstractToken.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Security\Core\Authentication\Token;
 
 use Symfony\Component\Security\Core\User\UserInterface;
+use Symfony\Component\Security\Core\User\InMemoryUser;
 
 /**
  * Base class for Token instances.
@@ -114,7 +115,8 @@ public function __serialize(): array
      */
     public function __unserialize(array $data): void
     {
-        [$this->user, , , $this->attributes, $this->roleNames] = $data;
+        [$user, , , $this->attributes, $this->roleNames] = $data;
+        $this->user = \is_string($user) ? new InMemoryUser($user, '', $this->roleNames) : $user;
     }
 
     /**