#207 Change Maven build image back

Changed the image back to maven 3.6 because the
buildfrontend-maven-plugin doesn't support 3.9.

Signed-off-by: Jeremias Weber <jeremias.weber@cloudogu.com>

Jenkinsfile

@@ -21,7 +21,7 @@ parallel(
     node() { // No specific label
       timestamps {
 
-        def mvnDockerName = '3.9-eclipse-temurin-17'
+        def mvnDockerName = '3.6-openjdk-17'
         Maven mvn = new MavenInDocker(this, mvnDockerName)
 
         stage('Checkout') {