Add missing @experimental annotations

src/Symfony/Component/Messenger/Handler/MessageHandlerInterface.php

@@ -15,6 +15,8 @@ namespace Symfony\Component\Messenger\Handler;
  * Marker interface for message handlers.
  *
  * @author Samuel Roze <samuel.roze@gmail.com>
+ *
+ * @experimental in 4.1
  */
 interface MessageHandlerInterface
 {

src/Symfony/Component/Messenger/Handler/MessageSubscriberInterface.php

@@ -15,6 +15,8 @@ namespace Symfony\Component\Messenger\Handler;
  * Handlers can implement this interface to handle multiple messages.
  *
  * @author Samuel Roze <samuel.roze@gmail.com>
+ *
+ * @experimental in 4.1
  */
 interface MessageSubscriberInterface extends MessageHandlerInterface
 {

src/Symfony/Component/Messenger/Transport/Factory/TransportFactoryInterface.php

@@ -18,6 +18,8 @@ use Symfony\Component\Messenger\Transport\SenderInterface;
  * Creates a Messenger transport.
  *
  * @author Samuel Roze <samuel.roze@gmail.com>
+ *
+ * @experimental in 4.1
  */
 interface TransportFactoryInterface
 {