Fix multi-release JAR test on JDK 15

See gh-21605

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

@@ -99,6 +99,7 @@ public abstract class TestJarCreator {
 			writeEntry(jarOutputStream, "META-INF/versions/12/multi-release.dat", 12);
 			writeEntry(jarOutputStream, "META-INF/versions/13/multi-release.dat", 13);
 			writeEntry(jarOutputStream, "META-INF/versions/14/multi-release.dat", 14);
+			writeEntry(jarOutputStream, "META-INF/versions/15/multi-release.dat", 15);
 		}
 		else {
 			writeEntry(jarOutputStream, "3.dat", 3);