minor #14797 [Messenger] Deprecated the prefetch_count option (javiereguiluz)

This PR was squashed before being merged into the 5.3-dev branch.

Discussion
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[Messenger] Deprecated the prefetch_count option

Fixes #14770.

Commits
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89d3122 [Messenger] Deprecated the prefetch_count option

Author: javiereguiluz

messenger.rst

@@ -1014,6 +1014,11 @@ The transport has a number of options:
 ``exchange[type]``                            Type of exchange                                   ``fanout``
 ============================================  =================================================  ===================================
 
+.. deprecated:: 5.3
+
+    The ``prefetch_count`` option was deprecated in Symfony 5.3 because it has
+    no effect on the AMQP Messenger transport.
+
 You can also configure AMQP-specific settings on your message by adding
 :class:`Symfony\\Component\\Messenger\\Bridge\\Amqp\\Transport\\AmqpStamp` to
 your Envelope::