Reduce method visibility in EndpointDiscovererTests

See gh-18884

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/annotation/EndpointDiscovererTests.java

@@ -504,7 +504,7 @@ class EndpointDiscovererTests {
 	static class SpecializedSuperclassTestEndpoint extends AbstractFilteredEndpoint {
 
 		@ReadOperation
-		public Object getAll() {
+		Object getAll() {
 			return null;
 		}