[CTable] 修改 CDropdown 层级

Author: liyuan-meng

src/components/c-dropdown/index.less

@@ -34,7 +34,7 @@
   width: fit-content;
   min-width: fit-content !important;
 
-  z-index: 100;
+  z-index: @z-index-dropdown;
 
   &-menu-area {
     border-radius: 6px;