Remove more references to 'model' in GPUI APIs (#23693)

Release Notes:

- N/A

crates/breadcrumbs/src/breadcrumbs.rs

@@ -152,7 +152,7 @@ impl ToolbarItemView for Breadcrumbs {
             return ToolbarItemLocation::Hidden;
         };
 
-        let this = cx.model().downgrade();
+        let this = cx.entity().downgrade();
         self.subscription = Some(item.subscribe_to_item_events(
             window,
             cx,