Guard javadoc -Werror behind JDK 15+

The -Werror flag for the javadoc tool was added in JDK 15
(https://bugs.openjdk.org/browse/JDK-8232513). On older JDKs javadoc
rejects the flag outright, which breaks any task that invokes javadoc
(javadocJar, and therefore publishToSonatype / publishToMavenLocal)
when Gradle is running on JDK 11.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: donald-pinckney

gradle/java.gradle

@@ -33,7 +33,11 @@ subprojects {
     javadoc {
         options.encoding = 'UTF-8'
         options.addStringOption('Xdoclint:reference', '-quiet')
-        options.addBooleanOption('Werror', true)
+        // -Werror for javadoc was added in JDK 15 (JDK-8232513). Skip on older JDKs
+        // otherwise the javadoc task rejects the flag (breaks the JDK 11 publish job).
+        if (JavaVersion.current().isCompatibleWith(JavaVersion.VERSION_15)) {
+            options.addBooleanOption('Werror', true)
+        }
         if (JavaVersion.current().isJava9Compatible()) {
             options.addBooleanOption('html5', true)
         }