Use v-tree8 as stylename for new Tree component (#9238)

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

     /**
      * String renderer that handles icon resources and stores their identifiers
      * into data objects.
-     * 
+     *
      * @since 8.1
      */
     public final class TreeRenderer extends AbstractRenderer<T, String>
         while (treeGrid.getHeaderRowCount() > 0) {
             treeGrid.removeHeaderRow(0);
         }
-        treeGrid.setPrimaryStyleName("v-newtree");
+        treeGrid.setPrimaryStyleName("v-tree8");
 
         setWidth("100%");
         treeGrid.setHeightUndefined();

testbench-api/src/main/java/com/vaadin/testbench/elements/TreeElement.java

      */
     public WebElement getExpandElement(int rowIndex) {
         return asTreeGrid().getCell(rowIndex, 0)
-                .findElement(By.className("v-newtree-expander"));
+                .findElement(By.className("v-tree8-expander"));
 
     }
 

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

 @import "textarea";
 @import "tree";
 @import "treegrid";
-@import "newtree";
+@import "tree8";
 @import "treetable";
 @import "twincolselect";
 @import "upload";
     @include valo-tree;
   }
 
-  @if v-is-included(newtree) {
-    @include valo-newtree;
+  @if v-is-included(tree8) {
+    @include valo-tree8;
   }
 
   @if v-is-included(treegrid) {

themes/src/main/themes/VAADIN/themes/valo/components/_newtree.scss → themes/src/main/themes/VAADIN/themes/valo/components/_tree8.scss

 @import "treegrid";
 
-$v-newtree-border-radius: 3px;
+$v-tree8-border-radius: 3px;
 
-@mixin valo-newtree ($primary-stylename: v-newtree) {
+@mixin valo-tree8 ($primary-stylename: v-tree8) {
 
   @include valo-treegrid($primary-stylename);
 
   .#{$primary-stylename}-cell-content {
     border: $v-grid-cell-focused-border;
     border-color: transparent;
-    border-radius: $v-newtree-border-radius;
+    border-radius: $v-tree8-border-radius;
     padding: $v-grid-cell-padding;
   }
 

themes/src/main/themes/VAADIN/themes/valo/shared/_variables.scss

     textarea,
     richtextarea,
     tree,
-    newtree,
+    tree8,
     treegrid,
     treetable,
     twincolselect,