minor #20926 [Messenger] Fix instance variable declaration (MaximePinot)

This PR was merged into the 6.4 branch.

Discussion
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[Messenger] Fix instance variable declaration

Add missing `private` keyword to use constructor property promotion. Otherwise, the call to `$this->em` in the `__invoke` method is invalid, as `$em` is considered a constructor parameter only.

Commits
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34df2c8 [Messenger] Fix instance variable declaration

Author: javiereguiluz

messenger.rst

@@ -2548,7 +2548,7 @@ using the ``DispatchAfterCurrentBusMiddleware`` and adding a
     {
         public function __construct(
             private MailerInterface $mailer,
-            EntityManagerInterface $em,
+            private EntityManagerInterface $em,
         ) {
         }