Update 3.12.0 release notes

Author: michaelklishin

release-notes/3.12.0.md

@@ -243,6 +243,12 @@ This release includes all bug fixes shipped in the `3.11.x` series.
 
    GitHub issue: [#7743](https://github.com/rabbitmq/rabbitmq-server/issues/7743)
 
+ * Quorum queues now have a mechanism of shrinking down to just one replica.
+   This is meant to be used **exclusively** for disaster recovery when a majority
+   of nodes hosting replicas of a queue were permanently lost.
+
+   GitHub issue: [#8322](https://github.com/rabbitmq/rabbitmq-server/pull/8322)
+
  * Classic mirrored queues that had a [length limit](https://rabbitmq.com/maxlength.html) defined on them
    handled the overflow of messages differently from the current elected leader,
    eventually causing exceptions and replica restarts.