Mention #5077 in 3.13.0 release notes

release-notes/3.13.0.md

@@ -87,6 +87,18 @@ Release notes are kept under [rabbitmq-server/release-notes](https://github.com/
 
 #### Enhancements
 
+ * Messages are now internally stored using a new common heavily AMQP 1.0-influenced container format. This is a major step towards a protocol-agnostic core:
+   a common format that encapsulates a sum of data types used by the protocols RabbitMQ supports, plus annotations for routng, dead-lettering state,
+   and other purposes. 
+
+   AMQP 1.0, AMQP 0-9-1, MQTT and STOMP have or will adopt this internal representation in upcoming releases. RabbitMQ Stream protocol already uses the AMQP 1.0 message container
+   structure internally.
+
+   This common internal format will allow for more correct and potentially efficient multi-protocol support in RabbitMQ,
+   and that most cross-protocol translation rough edges can be smoothened.
+
+   GitHub issue: [#5077](https://github.com/rabbitmq/rabbitmq-server/pull/5077)
+
  * Target quorum queue replica state is now continuously reconciled.
 
    When the number of online replicas of a quorum queue goes below (or above) its target,