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

Release Notes:

- N/A

crates/activity_indicator/src/activity_indicator.rs

@@ -473,7 +473,7 @@ impl Render for ActivityIndicator {
         let Some(content) = self.content_to_render(cx) else {
             return result;
         };
-        let this = cx.model().downgrade();
+        let this = cx.entity().downgrade();
         let truncate_content = content.message.len() > MAX_MESSAGE_LEN;
         result.gap_2().child(
             PopoverMenu::new("activity-indicator-popover")