minor #29381 Added upgrade to HandlersLocator (alexander-schranz)

This PR was squashed before being merged into the 4.2 branch (closes #29381).

Discussion
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Added upgrade to HandlersLocator

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | missing upgrade
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no tests
| Fixed tickets | fixes #29367
| License       | MIT

Added upgrade to HandlersLocator

Commits
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d4edb1b966 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  * The `EncoderInterface` and `DecoderInterface` interfaces have been replaced by a unified `Symfony\Component\Messenger\Transport\Serialization\SerializerInterface`.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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