Tighten type bounds on GeneratedMessageLite.defaultInstanceMap.

Just a cleanup I noticed while looking at optimising this code.

PiperOrigin-RevId: 662234161

java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java

@@ -360,8 +360,8 @@ public abstract class GeneratedMessageLite<
     return dynamicMethod(MethodToInvoke.BUILD_MESSAGE_INFO);
   }
 
-  private static Map<Object, GeneratedMessageLite<?, ?>> defaultInstanceMap =
+  private static Map<Class<?>, GeneratedMessageLite<?, ?>> defaultInstanceMap =
-      new ConcurrentHashMap<Object, GeneratedMessageLite<?, ?>>();
+      new ConcurrentHashMap<>();
 
   @SuppressWarnings("unchecked")
   static <T extends GeneratedMessageLite<?, ?>> T getDefaultInstance(Class<T> clazz) {