feat: proxy to isPropagationStopped

Modifies Event::propagationIsStopped such that it now proxies to the
isPropagationStopped method, and documents in the deprecation notice
that this happens.

Author: weierophinney

src/Event.php

@@ -191,12 +191,14 @@ public function stopPropagation($flag = true)
      * Is propagation stopped?
      *
      * @deprecated Use isPropagationStopped instead, to make your application
-     *     forwards-compatible with PSR-14 and zend-eventmanager v4.
+     *     forwards-compatible with PSR-14 and zend-eventmanager v4. If you
+     *     plan to override this method, please do so via the `isPropagationStopped`
+     *     method, as this method proxies to that one starting in version 3.3.0.
      * @return bool
      */
     public function propagationIsStopped()
     {
-        return $this->stopPropagation;
+        return $this->isPropagationStopped();
     }
 
     /**