Update to OpenJDK 15.

Change-Id: I1ec0958b83b5250da1fa855e3754228e11f132f5
diff --git a/src/main/resources/application.properties b/src/main/resources/application.properties
index e966bda..3d56d21 100644
--- a/src/main/resources/application.properties
+++ b/src/main/resources/application.properties
@@ -70,7 +70,7 @@
 quarkus.container-image.name = mulkcms2
 quarkus.container-image.registry = docker.benkard.de
 
-quarkus.jib.base-jvm-image = openjdk:14
+quarkus.jib.base-jvm-image = openjdk:15-slim
 #quarkus.jib.jvm-arguments = -XX:+UnlockExperimentalVMOptions,-XX:+UseShenandoahGC,-XX:ShenandoahGCHeuristics=compact
 
 quarkus.native.container-runtime = docker