fix(grid): Update grid cell active state selector specificity (#15402)

Author: desig9stein

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

@@ -1791,8 +1791,8 @@
     %grid-cell--active {
         box-shadow: inset 0 0 0 rem(1px) var-get($theme, 'cell-active-border-color');
 
-        %igx-grid__filtering-cell,
-        %grid-cell-header {
+        > %igx-grid__filtering-cell,
+        > %grid-cell-header {
             border-inline-end-color: var-get($theme, 'cell-active-border-color');
             border-bottom-color: var-get($theme, 'cell-active-border-color');
         }