Update multi release test jar to include Java 12

See gh-16970

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/TestJarCreator.java

@@ -95,6 +95,7 @@ public abstract class TestJarCreator {
 			writeEntry(jarOutputStream, "META-INF/versions/10/multi-release.dat", 10);
 			writeEntry(jarOutputStream, "META-INF/versions/11/multi-release.dat", 11);
 			writeEntry(jarOutputStream, "META-INF/versions/12/multi-release.dat", 12);
+			writeEntry(jarOutputStream, "META-INF/versions/13/multi-release.dat", 13);
 		}
 		else {
 			writeEntry(jarOutputStream, "3.dat", 3);