[JENKINS-32978] Support latest source version to avoid compile time warnings with JDK7 (#2032)

see http://stackoverflow.com/questions/8185331/forward-compatible-java-6-annotation-processor-and-supportedsourceversion

core/src/main/java/jenkins/PluginSubtypeMarker.java

@@ -30,7 +30,6 @@ import javax.annotation.processing.AbstractProcessor;
 import javax.annotation.processing.Processor;
 import javax.annotation.processing.RoundEnvironment;
 import javax.annotation.processing.SupportedAnnotationTypes;
-import javax.annotation.processing.SupportedSourceVersion;
 import javax.lang.model.SourceVersion;
 import javax.lang.model.element.Element;
 import javax.lang.model.element.ElementKind;
@@ -54,7 +53,6 @@ import java.util.Set;
  * @author Kohsuke Kawaguchi
  * @since 1.420
  */
-@SupportedSourceVersion(SourceVersion.RELEASE_6)
 @SupportedAnnotationTypes("*")
 @MetaInfServices(Processor.class)
 @SuppressWarnings({"Since15"})
@@ -80,6 +78,11 @@ public class PluginSubtypeMarker extends AbstractProcessor {
 
                     return super.visitType(e, aVoid);
                 }
+
+                @Override
+                public Void visitUnknown(Element e, Void aVoid) {
+                    return DEFAULT_VALUE;
+                }
             };
 
             for (Element e : roundEnv.getRootElements()) {
@@ -100,6 +103,11 @@ public class PluginSubtypeMarker extends AbstractProcessor {
         }
     }
 
+    @Override
+    public SourceVersion getSupportedSourceVersion() {
+        return SourceVersion.latest();
+    }
+
     private void write(TypeElement c) throws IOException {
         FileObject f = processingEnv.getFiler().createResource(StandardLocation.CLASS_OUTPUT,
                 "", "META-INF/services/hudson.Plugin");