Annotate isJavaWebStartSupported as no external use

Should only be used by web page.

Author: MarkEWaite

core/src/main/java/hudson/slaves/JNLPLauncher.java

@@ -232,6 +232,7 @@ public boolean filterType(@Nonnull Class<?> contextClass, @Nonnull Descriptor de
      * @return {@code true} if Java Web Start button should be displayed.
      * @since FIXME
      */
+    @Restricted(NoExternalUse.class) // Jelly use
     public boolean isJavaWebStartSupported() {
         return System.getProperty("java.version", "1.8").startsWith("1.8");
     }