fix: fix table container z-index (#5786)

Author: shunyue1320

components/table/style/index.less

@@ -642,7 +642,7 @@
       position: absolute;
       top: 0;
       bottom: 0;
-      z-index: 1;
+      z-index: @zindex-table-fixed;
       width: 30px;
       transition: box-shadow 0.3s;
       content: '';