Add missing supported source version

Closes gh-10929

Author: snicoll

spring-boot-tools/spring-boot-autoconfigure-processor/src/main/java/org/springframework/boot/autoconfigureprocessor/AutoConfigureAnnotationProcessor.java

@@ -29,6 +29,7 @@
 import javax.annotation.processing.AbstractProcessor;
 import javax.annotation.processing.RoundEnvironment;
 import javax.annotation.processing.SupportedAnnotationTypes;
+import javax.lang.model.SourceVersion;
 import javax.lang.model.element.AnnotationMirror;
 import javax.lang.model.element.AnnotationValue;
 import javax.lang.model.element.Element;
@@ -80,6 +81,11 @@ protected void addAnnotations(Map<String, String> annotations) {
 				"org.springframework.boot.autoconfigure.AutoConfigureOrder");
 	}
 
+	@Override
+	public SourceVersion getSupportedSourceVersion() {
+		return SourceVersion.latestSupported();
+	}
+
 	@Override
 	public boolean process(Set<? extends TypeElement> annotations,
 			RoundEnvironment roundEnv) {