Merge pull request #4453 from rabbitmq/mk-3.9.15-release-notes-update

Update 3.9.15 release notes

(cherry picked from commit b44c649)

Author: michaelklishin

release-notes/3.9.15.md

@@ -41,6 +41,15 @@ Contributors are encouraged to update them together with their changes. This hel
    GitHub issue: [#4408](https://github.com/rabbitmq/rabbitmq-server/pull/4408)
 
 
+### LDAP Plugin
+
+#### Enhancements
+
+  * More Erlang 24.3's `eldap` library compatibility improvements.
+
+    GitHub issue: [#4448](https://github.com/rabbitmq/rabbitmq-server/pull/4448)
+
+
 ## Dependency Upgrades
 
  * `ra` upgraded [from `2.0.4` to `2.0.6`](https://github.com/rabbitmq/ra/compare/v2.0.4...v2.0.6)