MC-4262: Column Rearranging Drop Zone Indicators Are Too Small & Sometimes Hard To Activate

- Fix zIndex issues

Author: davemacaulay

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/column-group/_default.less

@@ -60,7 +60,7 @@
             transition: .25s opacity;
             visibility: hidden;
             width: 6px;
-            z-index: 10;
+            z-index: 26;
 
             .no-user-select;
             .no-pointer-events;