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/language/src/buffer.rs

@@ -928,7 +928,7 @@ impl Buffer {
         language_registry: Option<Arc<LanguageRegistry>>,
         cx: &mut App,
     ) -> impl Future<Output = BufferSnapshot> {
-        let entity_id = cx.reserve_model::<Self>().entity_id();
+        let entity_id = cx.reserve_entity::<Self>().entity_id();
         let buffer_id = entity_id.as_non_zero_u64().into();
         async move {
             let text =