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

Release Notes:

- N/A

crates/welcome/src/base_keymap_picker.rs

@@ -30,7 +30,7 @@ pub fn toggle(
     let fs = workspace.app_state().fs.clone();
     workspace.toggle_modal(window, cx, |window, cx| {
         BaseKeymapSelector::new(
-            BaseKeymapSelectorDelegate::new(cx.model().downgrade(), fs, cx),
+            BaseKeymapSelectorDelegate::new(cx.entity().downgrade(), fs, cx),
             window,
             cx,
         )

crates/welcome/src/multibuffer_hint.rs

@@ -109,7 +109,7 @@ impl ToolbarItemView for MultibufferHint {
             return ToolbarItemLocation::Hidden;
         };
 
-        let this = cx.model().downgrade();
+        let this = cx.entity().downgrade();
         self.subscription = Some(active_pane_item.subscribe_to_item_events(
             window,
             cx,