Rename model based variable names to entity (#24198)

## Context
While looking through the client crate, I noticed that some of the old
functions and variables were still using gpui::model name that was
deprecated during the gpui3 transition. This PR renames those instances
of model to entity to be more inline with gpui3.

In addition, I also renamed `model` to `entity` in cases found by the
below search terms given by @someone13574

- model = cx.
- model: Entity
- model: &Entity
- OpenedModelHandle
- model.update
- model.upgrade
- model = .*\.root (regex)
- parent_model
- model = cx.new
- cx.spawn(move |model

Release Notes:

- N/A

crates/project/src/debounced_delay.rs

@@ -35,7 +35,7 @@ impl<E: 'static> DebouncedDelay<E> {
         self.cancel_channel = Some(sender);
 
         let previous_task = self.task.take();
-        self.task = Some(cx.spawn(move |model, mut cx| async move {
+        self.task = Some(cx.spawn(move |entity, mut cx| async move {
             let mut timer = cx.background_executor().timer(delay).fuse();
             if let Some(previous_task) = previous_task {
                 previous_task.await;
@@ -46,7 +46,7 @@ impl<E: 'static> DebouncedDelay<E> {
                 _ = timer => {}
             }
 
-            if let Ok(task) = model.update(&mut cx, |project, cx| (func)(project, cx)) {
+            if let Ok(task) = entity.update(&mut cx, |project, cx| (func)(project, cx)) {
                 task.await;
             }
         }));