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/project_symbols/src/project_symbols.rs

@@ -350,7 +350,7 @@ mod tests {
             cx.add_window_view(|window, cx| Workspace::test_new(project.clone(), window, cx));
 
         // Create the project symbols view.
-        let symbols = cx.new_window_model(|window, cx| {
+        let symbols = cx.new_window_entity(|window, cx| {
             Picker::uniform_list(
                 ProjectSymbolsDelegate::new(workspace.downgrade(), project.clone()),
                 window,