Merge pull request #5704 from IgniteUI/mvenkov/set-grid-z-index-to-1-7.3.x

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

Author: kdinev

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

@@ -570,7 +570,7 @@
         overflow: hidden;
         box-shadow: $grid-shadow;
         outline-style: none;
-        z-index: 0;
+        z-index: 1;
 
         %cbx-display {
             min-width: rem(20px);