Make it clear that 4.0.0 will go through betas and RCs first

Author: michaelklishin

release-notes/4.0.0.md

@@ -1,6 +1,6 @@
-## RabbitMQ 4.0.0
+## RabbitMQ 4.0.0-beta.3
 
-RabbitMQ `4.0.0` is a new major release.
+RabbitMQ `4.0.0-beta.3` is a preview of a new major release.
 
 Starting June 1st, 2024, community support for this series will only be provided to [regularly contributing users](https://github.com/rabbitmq/rabbitmq-server/blob/main/COMMUNITY_SUPPORT.md) and those who hold a valid [commercial support license](https://tanzu.vmware.com/rabbitmq/oss).