fix: Cannot delete empty column inside a tab using the dashboard edit…

…or (#29346)

(cherry picked from commit ee52277)

superset-frontend/src/dashboard/components/menu/HoverMenu.tsx

@@ -32,7 +32,7 @@ const HoverStyleOverrides = styled.div`
   .hover-menu {
     opacity: 0;
     position: absolute;
-    z-index: 10;
+    z-index: 11; // one more than DragDroppable
     font-size: ${({ theme }) => theme.typography.sizes.m};
   }