Replace explicit generics with diamond operator

Where possible, explicit generic declarations to use the Java 8 diamond
operator.

See gh-9781

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/properties/AnnotationsPropertySource.java

@@ -60,7 +60,7 @@ public class AnnotationsPropertySource extends EnumerablePropertySource<Class<?>
 
 	private Map<String, Object> getProperties(Class<?> source) {
 		Map<String, Object> properties = new LinkedHashMap<>();
-		collectProperties(source, source, properties, new HashSet<Class<?>>());
+		collectProperties(source, source, properties, new HashSet<>());
 		return Collections.unmodifiableMap(properties);
 	}