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/zeta/src/zeta.rs

@@ -230,9 +230,9 @@ impl Zeta {
         cx: &mut App,
     ) -> Entity<Self> {
         let this = Self::global(cx).unwrap_or_else(|| {
-            let model = cx.new(|cx| Self::new(client, user_store, cx));
-            cx.set_global(ZetaGlobal(model.clone()));
-            model
+            let entity = cx.new(|cx| Self::new(client, user_store, cx));
+            cx.set_global(ZetaGlobal(entity.clone()));
+            entity
         });
 
         this.update(cx, move |this, cx| {