feature #33152 Mark all dispatched event classes as final (Tobion)

This PR was merged into the 4.4 branch.

Discussion
----------

Mark all dispatched event classes as final

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |
| License       | MIT
| Doc PR        |

I think we should mark all our Event classes as final. There is no point in people extending them as the libraries that use the event, will only dispatch this event. So extending events in user-land achieves nothing as the subclasses won't be dispatched.
I'm not talking about the base events that are meant to be extended like KernelEvent, but the leaf events like ExceptionEvent, ResponseEvent etc.
Then we can also make them real final in 5.0 as the events are value objects that should not be mocked.

Commits
-------

4bb38eec89 Mark all dispatched event classes as final

Author: nicolas-grekas

Event/AuthenticationFailureEvent.php

@@ -18,6 +18,8 @@
  * This event is dispatched on authentication failure.
  *
  * @author Johannes M. Schmitt <[email protected]>
+ *
+ * @final since Symfony 4.4
  */
 class AuthenticationFailureEvent extends AuthenticationEvent
 {

Event/AuthenticationSuccessEvent.php

@@ -11,6 +11,9 @@
 
 namespace Symfony\Component\Security\Core\Event;
 
+/**
+ * @final since Symfony 4.4
+ */
 class AuthenticationSuccessEvent extends AuthenticationEvent
 {
 }

Event/VoteEvent.php

@@ -21,7 +21,7 @@
  *
  * @internal
  */
-class VoteEvent extends Event
+final class VoteEvent extends Event
 {
     private $voter;
     private $subject;