Update 3.9.14 release notes to include #4324

(cherry picked from commit 4be125d)

Author: michaelklishin

release-notes/3.9.14.md

@@ -47,6 +47,12 @@ Contributors are encouraged to update them together with their changes. This hel
 
    GitHub issue: [#4256](https://github.com/rabbitmq/rabbitmq-server/pull/4256)
 
+ * File handle cache efficiency improvements.
+
+   Contributed by @felixhuettner.
+
+   GitHub issue: [#4324](https://github.com/rabbitmq/rabbitmq-server/pull/4324)
+
  * Uknown stream properties (e.g. those requested by a node that runs a newer version)
    are now handled gracefully.