increase number of styles for indented TreeGrid rows, fixes #11358 (#11392)

server/src/main/java/com/vaadin/ui/TreeGrid.java

 /**
  * A grid component for displaying hierarchical tabular data.
  *
+ * Visual hierarchy depth positioning of rows is done via styles, see
+ * <code>_treegrid.scss</code> from Valo theme.
+ *
  * @author Vaadin Ltd
  * @since 8.1
  *

themes/src/main/themes/VAADIN/themes/valo/components/_treegrid.scss

 
   // Hierarchy depth styling
   .#{$primary-stylename}-node {
-    @for $i from 0 through 15 {
+    // see https://github.com/vaadin/framework/issues/11358
+    @for $i from 0 through 31 {
       &.#{$v-treegrid-class-depth}-#{$i} {
         padding-left: $v-treegrid-indent * $i;
       }