Use isJava9Compatible as a property in the root-level build.gradle.kts file

build.gradle.kts

@@ -52,7 +52,7 @@ subprojects {
             // The following lines are needed at least for the current version of proguard
             dontwarn("java.**")
             val homeDir = System.getProperty("java.home") as String
-            if (JavaVersion.current().isJava9Compatible()) {
+            if (JavaVersion.current().isJava9Compatible) {
                 libraryjars("$homeDir/jmods/java.base.jmod")
             } else libraryjars("$homeDir/lib/rt.jar")
             // Note: dontpreverify() should NOT be used, it will cause errors at runtime