Update getSupportedSourceVersion() logic

Use SourceVersion.latestSupported() rather than @SupportedSourceVersion
since we might build with an earlier JDK.

Fixes gh-2302

Author: philwebb

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

@@ -34,7 +34,6 @@
 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.AnnotationMirror;
 import javax.lang.model.element.AnnotationValue;
@@ -66,7 +65,6 @@
  * @since 1.2.0
  */
 @SupportedAnnotationTypes({ "*" })
-@SupportedSourceVersion(SourceVersion.RELEASE_8)
 public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor {
 
 	static final String CONFIGURATION_PROPERTIES_ANNOTATION = "org.springframework.boot."
@@ -101,6 +99,11 @@ protected String nestedConfigurationPropertyAnnotation() {
 		return NESTED_CONFIGURATION_PROPERTY_ANNOTATION;
 	}
 
+	@Override
+	public SourceVersion getSupportedSourceVersion() {
+		return SourceVersion.latestSupported();
+	}
+
 	@Override
 	public synchronized void init(ProcessingEnvironment env) {
 		super.init(env);