docs: update release notes with OLM warning (#816)

Author: razvan

modules/ROOT/partials/release-notes/release-25.11.adoc

@@ -786,6 +786,13 @@ These OpenShift versions are no longer supported:
 * `4.17`
 * `4.16`
 
+[WARNING]
+====
+There is a known issue when updating the certified Stackable Listener Operator on OpenShift clusters `4.18` from version `25.7.0` to `25.11.0`.
+In this case, the operator is unable to reconcile existing ListenerClass resources after the update.
+As a workaround, please uninstall the Stackable Listener Operator and reinstall the version `25.11.0` again directly.
+====
+
 ==== Upgrade from 25.7
 
 ===== Using stackablectl