Remove syn 0.15 from our crate graph (fixes #24421)

This required bumps of:

* gleam
* image
* rust-webvr
* webrender
* webxr

components/layout_thread_2020/lib.rs

@@ -1420,7 +1420,7 @@ impl LayoutThread {
             fragment_tree.print();
         }
         if self.dump_display_list {
-            display_list.wr.print_display_list();
+            display_list.wr.dump_serialized_display_list();
         }
 
         debug!("Layout done!");