Fix missed renames in #22632 (#23688)

Fix a bug where a GPUI macro still used `ModelContext`
Rename `AsyncAppContext` -> `AsyncApp`
Rename update_model, read_model, insert_model, and reserve_model to
update_entity, read_entity, insert_entity, and reserve_entity

Release Notes:

- N/A

crates/vim/src/test/vim_test_context.rs

@@ -94,7 +94,7 @@ impl VimTestContext {
         Self { cx }
     }
 
-    pub fn update_model<F, T, R>(&mut self, model: Entity<T>, update: F) -> R
+    pub fn update_entity<F, T, R>(&mut self, model: Entity<T>, update: F) -> R
     where
         T: 'static,
         F: FnOnce(&mut T, &mut Window, &mut Context<T>) -> R + 'static,