Mention Ra bump in 3.12.2 release notes

(cherry picked from commit 9d79599)

Author: michaelklishin

release-notes/3.12.2.md

@@ -163,7 +163,7 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
 
 ## Dependency Upgrades
 
-None in this release.
+ * `ra` was upgraded to [`2.6.3`](https://github.com/rabbitmq/ra/releases)
 
 ## Source Code Archives