update Java versions in README

Author: maxlaverse

README.md

@@ -22,7 +22,7 @@ node {
 
 ## Prerequisites
 
-- A jenkins installation running version 2.401.1 or higher (with jdk11 or jdk17).
+- A jenkins installation running version 2.401.1 or higher (with jdk17 or jdk21).
 - An executor with `kubectl` installed (tested against [v1.22 to v1.32][Jenkinsfile] included).
 - A Kubernetes cluster.