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/gpui/docs/contexts.md

@@ -24,7 +24,7 @@ Provides access to the global application state. All other kinds of contexts ult
 
 Provides access to the state of an application window, and also derefs to an `AppContext`, so you can pass a window context reference to any method taking an app context. Obtain this context by calling `WindowHandle::update`.
 
-## `ModelContext<T>`
+## `Context<T>`
 
 Available when you create or update a `Model<T>`. It derefs to an `AppContext`, but also contains methods specific to the particular model, such as the ability to notify change observers or emit events.
 
@@ -32,7 +32,7 @@ Available when you create or update a `Model<T>`. It derefs to an `AppContext`,
 
 Available when you create or update a `View<V>`. It derefs to a `WindowContext`, but also contains methods specific to the particular view, such as the ability to notify change observers or emit events.
 
-## `AsyncAppContext` and `AsyncWindowContext`
+## `AsyncApp` and `AsyncWindowContext`
 
 Whereas the above contexts are always passed to your code as references, you can call `to_async` on the reference to create an async context, which has a static lifetime and can be held across `await` points in async code. When you interact with `Model`s or `View`s with an async context, the calls become fallible, because the context may outlive the window or even the app itself.
 

crates/gpui/docs/key_dispatch.md

@@ -42,9 +42,9 @@ To bind actions, chain `on_action` on to your element:
 
 ```rust
 impl Render for Menu {
-    fn render(&mut self, window: &mut Window, cx: &mut ModelContext<Self>) -> impl Component {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl Component {
         div()
-            .on_action(|this: &mut Menu, move: &MoveUp, window: &mut Window, cx: &mut ModelContext<Menu>| {
+            .on_action(|this: &mut Menu, move: &MoveUp, window: &mut Window, cx: &mut Context<Menu>| {
                 // ...
             })
             .on_action(|this, move: &MoveDown, cx| {
@@ -59,10 +59,10 @@ In order to bind keys to actions, you need to declare a _key context_ for part o
 
 ```rust
 impl Render for Menu {
-    fn render(&mut self, window: &mut Window, cx: &mut ModelContext<Self>) -> impl Component {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl Component {
         div()
             .key_context("menu")
-            .on_action(|this: &mut Menu, move: &MoveUp, window: &mut Window, cx: &mut ModelContext<Menu>| {
+            .on_action(|this: &mut Menu, move: &MoveUp, window: &mut Window, cx: &mut Context<Menu>| {
                 // ...
             })
             .on_action(|this, move: &MoveDown, cx| {