refactor : NotificationEvent 객체를 생성하지 못하도록 추상 클래스로 변경

#663

backend/emm-sale/src/main/java/com/emmsale/event_publisher/NotificationEvent.java

@@ -7,7 +7,7 @@
 
 @RequiredArgsConstructor
 @Getter
-public class NotificationEvent {
+public abstract class NotificationEvent {
 
   @JsonIgnore
   private final Long receiverId;