minor #29366 [Messenger] Mention HandleTrait in UPGRADE-4.2.md file (ogizanagi)

This PR was merged into the 4.2 branch.

Discussion
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[Messenger] Mention HandleTrait in UPGRADE-4.2.md file

| Q             | A
| ------------- | ---
| Branch?       | 4.2 <!-- see below -->
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | N/A   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | N/A

Mentions this trait as a solution to build query buses.
I've removed the callable suggestion, as I suspect this won't be the most useful one.

Commits
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3cd4477563 [Messenger] Mention HandleTrait in UPGRADE-4.2.md file

UPGRADE-4.2.md

@@ -199,7 +199,10 @@ Messenger
 ---------
 
  * The `MiddlewareInterface::handle()` and `SenderInterface::send()` methods must now return an `Envelope` instance.
- * The return value of handlers is ignored. If you used to return a value, e.g in query bus handlers, you can either:
+ * The return value of handlers isn't forwarded anymore by middleware and buses. 
+   If you used to return a value, e.g in query bus handlers, you can either:
+    - get the result from the `HandledStamp` in the envelope returned by the bus.
+    - use the `HandleTrait` to leverage a message bus, expecting a single, synchronous message handling and returning its result.
     - make your `Query` mutable to allow setting & getting a result:
       ```php
       // When dispatching:
@@ -209,15 +212,6 @@ Messenger
       // In your handler:
       $query->setResult($yourResult);
       ```
-    - define a callable on your `Query` to be called in your handler:
-      ```php
-      // When dispatching:
-      $bus->dispatch(new Query([$this, 'onResult']));
-
-      // In your handler:
-      $query->executeCallback($yourResult);
-      ```
-
  * The `EnvelopeAwareInterface` was removed and the `MiddlewareInterface::handle()` method now requires an `Envelope` object
    as first argument. When using built-in middleware with the provided `MessageBus`, you will not have to do anything.  
    If you use your own `MessageBusInterface` implementation, you must wrap the message in an `Envelope` before passing it to middleware.