Properly set accessible value on label nodes (bevyengine#16418)

# Objective

AccessKit/accesskit#475 changed how text content
should be set for AccessKit nodes with a role of `Label`. This was
unfortunately missing from bevyengine#16234.

## Solution

When building an `accesskit::Node` with `Role::Label`, calls `set_value`
instead of `set_label` on the node to set its content.

## Testing

I can't test this right now on my Windows machine due to a compilation
error with wgpu-hal I have no idea how to resolve.

crates/bevy_ui/src/accessibility.rs

@@ -133,14 +133,14 @@ fn label_changed(
         if let Some(mut accessible) = accessible {
             accessible.set_role(Role::Label);
             if let Some(label) = label {
-                accessible.set_label(label);
+                accessible.set_value(label);
             } else {
-                accessible.clear_label();
+                accessible.clear_value();
             }
         } else {
             let mut node = Node::new(Role::Label);
             if let Some(label) = label {
-                node.set_label(label);
+                node.set_value(label);
             }
             commands
                 .entity(entity)