refactor deprecated methodes

Security/User/KeycloakBearerUser.php

@@ -245,6 +245,14 @@ class KeycloakBearerUser implements UserInterface, \Serializable
         return $this->preferred_username;
     }
 
+    /**
+     * @return string
+     */
+    public function getUserIdentifier(): string
+    {
+        return $this->preferred_username;
+    }
+
     /**
      * Removes sensitive data from the user.
      *
@@ -298,4 +306,4 @@ class KeycloakBearerUser implements UserInterface, \Serializable
             $this->accessToken
             ) = unserialize($serialized, ['allowed_classes' => false]);
     }
-}
+}