Revert "fix(Grid): set z-index to auto - to prevent the overlapping of esf dialog #12850 (#12994)" (#13118)

This reverts commit 0c2956a.

Author: ddincheva

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

@@ -770,7 +770,7 @@
         }
 
         outline-style: none;
-        z-index: auto;
+        z-index: 1;
 
         %cbx-display {
             min-width: $cbx-size;