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/ui/src/components/scrollbar.rs

@@ -114,7 +114,7 @@ impl ScrollbarState {
     }
 
     /// Set a parent model which should be notified whenever this Scrollbar gets a scroll event.
-    pub fn parent_model<V: 'static>(mut self, v: &Entity<V>) -> Self {
+    pub fn parent_entity<V: 'static>(mut self, v: &Entity<V>) -> Self {
         self.parent_id = Some(v.entity_id());
         self
     }