Update 3.12.1 release notes

(cherry picked from commit fa25f30)

Author: michaelklishin

release-notes/3.12.1.md

@@ -58,6 +58,10 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
 
    GitHub issue: [#8596](https://github.com/rabbitmq/rabbitmq-server/pull/8596)
 
+ * Proxy Protocol v2 LOCAL packets were not supported.
+
+   GitHub issue: [#8654](https://github.com/rabbitmq/rabbitmq-server/issues/8654)
+
 #### Enhancements
 
  * When a quorum queue does not find its local replica data files on boot, it will now log