refactor: 탈퇴 여부 확인을 MemberStatus이 처리

src/main/java/com/gdschongik/gdsc/domain/member/domain/Member.java

@@ -164,7 +164,7 @@ private void validateRegex(String value, String regex) {
     }
 
     public void validateStatus() {
-        if (this.status == MemberStatus.DELETED) {
+        if (isDeleted()) {
             throw new CustomException(MEMBER_DELETED);
         }
     }