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/workspace/src/item.rs

@@ -1504,8 +1504,8 @@ pub mod test {
             _window: &mut Window,
             cx: &mut App,
         ) -> Task<anyhow::Result<Entity<Self>>> {
-            let model = cx.new(|cx| Self::new_deserialized(workspace_id, cx));
-            Task::ready(Ok(model))
+            let entity = cx.new(|cx| Self::new_deserialized(workspace_id, cx));
+            Task::ready(Ok(entity))
         }
 
         fn cleanup(

crates/workspace/src/pane.rs

@@ -2540,8 +2540,10 @@ impl Pane {
         let navigate_backward = IconButton::new("navigate_backward", IconName::ArrowLeft)
             .icon_size(IconSize::Small)
             .on_click({
-                let model = cx.entity().clone();
-                move |_, window, cx| model.update(cx, |pane, cx| pane.navigate_backward(window, cx))
+                let entity = cx.entity().clone();
+                move |_, window, cx| {
+                    entity.update(cx, |pane, cx| pane.navigate_backward(window, cx))
+                }
             })
             .disabled(!self.can_navigate_backward())
             .tooltip({
@@ -2554,8 +2556,8 @@ impl Pane {
         let navigate_forward = IconButton::new("navigate_forward", IconName::ArrowRight)
             .icon_size(IconSize::Small)
             .on_click({
-                let model = cx.entity().clone();
-                move |_, window, cx| model.update(cx, |pane, cx| pane.navigate_forward(window, cx))
+                let entity = cx.entity().clone();
+                move |_, window, cx| entity.update(cx, |pane, cx| pane.navigate_forward(window, cx))
             })
             .disabled(!self.can_navigate_forward())
             .tooltip({