Extend RetryableException from Throwable interface

Let RetryableException extend from the Throwable interface to prevent errors in Psalm if catching it

lib/Doctrine/DBAL/Exception/RetryableException.php

@@ -2,9 +2,13 @@
 
 namespace Doctrine\DBAL\Exception;
 
+use Throwable;
+
 /**
  * Marker interface for all exceptions where retrying the transaction makes sense.
+ *
+ * @psalm-immutable
  */
-interface RetryableException
+interface RetryableException extends Throwable
 {
 }