Update 3.13.0 release notes

Author: michaelklishin

release-notes/3.13.0.md

@@ -1,6 +1,6 @@
-## RabbitMQ 3.13.0-beta.4
+## RabbitMQ 3.13.0-beta.5
 
-RabbitMQ `3.13.0-beta.4` is a preview of a new feature release.
+RabbitMQ `3.13.0-beta.5` is a preview of a new feature release.
 
 ## Highlights
 
@@ -10,6 +10,7 @@ The user-facing areas that have seen the biggest improvements in this release ar
 
  * Support for [consumer-side stream filtering](https://github.com/rabbitmq/rabbitmq-server/pull/8207)
  * [MQTTv5 support](https://blog.rabbitmq.com/posts/2023/07/mqtt5)
+ * A new [common message container format](https://github.com/rabbitmq/rabbitmq-server/pull/5077) used internally, based on the AMQP 1.0 message container format
  * Improved classic non-mirrored queue performance with message sizes larger than
    4 KiB (or a different customized CQ index embedding threshold)
  * Classic queues use version 2 of the storage implementation (CQv2).