Update z-index of block actions (#15329)

src/Umbraco.Web.UI.Client/src/less/variables.less

@@ -353,7 +353,7 @@
 // Try to avoid customizing these :)
 @zIndexEditor: 100;
 @zIndexTree: 100;
-@zIndexBlockActions: 500;
+@zIndexBlockActions: 90; // Should be less than "zIndexEditor" to stay behind editor overlay in infinite mode.
 @zindexDropdown: 1000;
 @zindexPopover: 1010;
 @zindexTooltip: 1030;