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/collab_ui/src/collab_panel.rs

@@ -239,14 +239,14 @@ impl CollabPanel {
             )
             .detach();
 
-            let model = cx.entity().downgrade();
+            let entity = cx.entity().downgrade();
             let list_state = ListState::new(
                 0,
                 gpui::ListAlignment::Top,
                 px(1000.),
                 move |ix, window, cx| {
-                    if let Some(model) = model.upgrade() {
-                        model.update(cx, |this, cx| this.render_list_entry(ix, window, cx))
+                    if let Some(entity) = entity.upgrade() {
+                        entity.update(cx, |this, cx| this.render_list_entry(ix, window, cx))
                     } else {
                         div().into_any()
                     }