Update @SupportedSourceVersion to RELEASE_8

Update the @SupportedSourceVersion annotation to RELEASE_8 since it
should indicate "the latest source version an annotation processor
supports".

Fixes gh-2302

Author: philwebb

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

@@ -64,7 +64,7 @@
  * @since 1.2.0
  */
 @SupportedAnnotationTypes({ "*" })
-@SupportedSourceVersion(SourceVersion.RELEASE_6)
+@SupportedSourceVersion(SourceVersion.RELEASE_8)
 public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor {
 
 	static final String CONFIGURATION_PROPERTIES_ANNOTATION = "org.springframework.boot."