fix(grids): set grids z-index to 1, #5674

Author: wnvko

projects/igniteui-angular/src/lib/core/styles/components/grid/_grid-theme.scss

@@ -603,7 +603,7 @@
         overflow: hidden;
         box-shadow: $grid-shadow;
         outline-style: none;
-        z-index: 0;
+        z-index: 1;
         border: $grid-border-width solid --var($theme, 'grid-border-color');
 
         %cbx-display {