修改时间工具类中的 `yyyy-MM-dd HH:mm:ss` 为常量，适用于需要用到常量的地方

Signed-off-by: hjk2008 <hjk2008ext@163.com>

ruoyi-common/ruoyi-common-core/src/main/java/com/ruoyi/common/core/utils/DateUtils.java

@@ -26,7 +26,7 @@ public class DateUtils extends org.apache.commons.lang3.time.DateUtils
 
     public static String YYYYMMDDHHMMSS = "yyyyMMddHHmmss";
 
-    public static String YYYY_MM_DD_HH_MM_SS = "yyyy-MM-dd HH:mm:ss";
+    public static final String YYYY_MM_DD_HH_MM_SS = "yyyy-MM-dd HH:mm:ss";
 
     private static String[] parsePatterns = {
             "yyyy-MM-dd", "yyyy-MM-dd HH:mm:ss", "yyyy-MM-dd HH:mm", "yyyy-MM",