Added upgrade to HandlersLocator

UPGRADE-4.2.md

@@ -285,6 +285,23 @@ Messenger
    ```
  * The `EncoderInterface` and `DecoderInterface` interfaces have been replaced by a unified `Symfony\Component\Messenger\Transport\Serialization\SerializerInterface`.
    Each interface method have been merged untouched into the `Serializer` interface, so you can simply merge your two implementations together and implement the new interface.
+ * The `HandlerLocator` class was replaced with `Symfony\Component\Messenger\Handler\HandlersLocator`.
+
+   Before:
+   ```php
+   new HandlerLocator([
+        YourMessage::class => $handlerCallable,
+   ]);
+   ```
+
+   After:
+   ```php
+   new HandlersLocator([
+        YourMessage::class => [
+            $handlerCallable,
+        ]
+   ]);
+   ```
 
 Monolog
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