minor #20889 [EventDispatcher] [RFC] feat: swap from event constant to event class (94noni)

This PR was submitted for the 7.2 branch but it was merged into the 6.4 branch instead.

Discussion
----------

[EventDispatcher] [RFC] feat: swap from event constant to event class

Hello, another thing I notice in the doc is the usage of old "string" via (constant) for the event dispatcher and listener/subscriber

do you think it is worth it in sf v7+ to only document the "new way" via event class which is the event dispatched internally?

thank !

Commits
-------

4f50290 feat: swap from event constant to event class

Author: javiereguiluz

Diff for: event_dispatcher.rst

@@ -246,22 +246,21 @@ methods could be called before or after the methods defined in other listeners
 and subscribers. To learn more about event subscribers, read :doc:`/components/event_dispatcher`.
 
 The following example shows an event subscriber that defines several methods which
-listen to the same ``kernel.exception`` event::
+listen to the same ``kernel.exception`` event which has an ``ExceptionEvent``::
 
     // src/EventSubscriber/ExceptionSubscriber.php
     namespace App\EventSubscriber;
 
     use Symfony\Component\EventDispatcher\EventSubscriberInterface;
     use Symfony\Component\HttpKernel\Event\ExceptionEvent;
-    use Symfony\Component\HttpKernel\KernelEvents;
 
     class ExceptionSubscriber implements EventSubscriberInterface
     {
         public static function getSubscribedEvents(): array
         {
             // return the subscribed events, their methods and priorities
             return [
-                KernelEvents::EXCEPTION => [
+                ExceptionEvent::class => [
                     ['processException', 10],
                     ['logException', 0],
                     ['notifyException', -10],