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/collab_ui/src/channel_view.rs

@@ -180,7 +180,7 @@ impl ChannelView {
                 })
             })?;
 
-            cx.new_window_model(|window, cx| {
+            cx.new_window_entity(|window, cx| {
                 let mut this = Self::new(
                     project,
                     weak_workspace,

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -5,7 +5,7 @@ use collections::HashSet;
 use editor::{AnchorRangeExt, CompletionProvider, Editor, EditorElement, EditorStyle};
 use fuzzy::{StringMatch, StringMatchCandidate};
 use gpui::{
-    AsyncAppContext, AsyncWindowContext, Context, Entity, Focusable, FontStyle, FontWeight,
+    AsyncApp, AsyncWindowContext, Context, Entity, Focusable, FontStyle, FontWeight,
     HighlightStyle, IntoElement, Render, Task, TextStyle, WeakEntity, Window,
 };
 use language::{
@@ -285,7 +285,7 @@ impl MessageEditor {
     }
 
     async fn resolve_completions_for_candidates(
-        cx: &AsyncAppContext,
+        cx: &AsyncApp,
         query: &str,
         candidates: &[StringMatchCandidate],
         range: Range<Anchor>,