refactor(core): 将logger变量声明为final

- 修改logger变量为不可变的final类型
- 提高代码的可读性和安全性
- 避免意外重新赋值的可能性
- 符合Java最佳实践标准
- 确保日志记录器在类中的唯一性
- 增强代码维护性与稳定性

xxl-job-executor-samples/xxl-job-executor-sample-frameless/src/main/java/com/xxl/job/executor/sample/frameless/XxlJobFramelessApplication.java

@@ -10,7 +10,7 @@ import java.util.concurrent.TimeUnit;
  * @author xuxueli 2018-10-31 19:05:43
  */
 public class XxlJobFramelessApplication {
-    private static Logger logger = LoggerFactory.getLogger(XxlJobFramelessApplication.class);
+    private static final Logger logger = LoggerFactory.getLogger(XxlJobFramelessApplication.class);
 
     public static void main(String[] args) {