Fix generating JavaDoc when running build with Java 9 or higher

Otherwise "current" is JDK 11, but "javadoc" from JDK 8 is executed

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1910299 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/build.gradle

@@ -232,9 +232,7 @@ javadoc {
     failOnError = true
     doFirst {
         options {
-            if (JavaVersion.current().isJava9Compatible()) {
-                addBooleanOption('html5', true)
-            }
+            if (jdkVersion > 8) addBooleanOption('html5', true)
             links 'https://poi.apache.org/apidocs/dev/'
             links 'https://docs.oracle.com/javase/8/docs/api/'
             use = true