minor #14866 Updated UPGRADE-2.4.md (mickaelandrieu)

This PR was submitted for the 2.8 branch but it was merged into the 2.6 branch instead (closes #14866).

Discussion
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Updated UPGRADE-2.4.md

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [yes ?]
| Tests pass?   | [yes]
| Fixed tickets |
| License       | MIT
| Doc PR        |

A missing deprecation in the upgrade guide.

Commits
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b631a56 Updated UPGRADE-2.4.md

UPGRADE-2.4.md

@@ -7,3 +7,44 @@ Form
  * The constructor parameter `$precision` in `IntegerToLocalizedStringTransformer`
    is now ignored completely, because a precision does not make sense for
    integers.
+
+EventDispatcher
+----------------
+
+ * The `getDispatcher()` and `getName()` methods from `Symfony\Component\EventDispatcher\Event`
+   are deprecated, the event dispatcher instance and event name can be received in the listener call instead.
+
+    Before:
+
+    ```php
+    use Symfony\Component\EventDispatcher\Event;
+
+    class Foo
+    {
+        public function myFooListener(Event $event)
+        {
+            $dispatcher = $event->getDispatcher();
+            $eventName = $event->getName();
+            $dispatcher->dispatch('log', $event);
+
+            // ... more code
+       }
+    }
+    ```
+
+    After:
+
+    ```php
+    use Symfony\Component\EventDispatcher\Event;
+    use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+
+    class Foo
+    {
+        public function myFooListener(Event $event, $eventName, EventDispatcherInterface $dispatcher)
+        {
+            $dispatcher->dispatch('log', $event);
+
+            // ... more code
+        }
+    }
+    ```