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/project_settings.rs

@@ -240,7 +240,7 @@ pub struct SettingsObserver {
 /// upstream.
 impl SettingsObserver {
     pub fn init(client: &AnyProtoClient) {
-        client.add_model_message_handler(Self::handle_update_worktree_settings);
+        client.add_entity_message_handler(Self::handle_update_worktree_settings);
     }
 
     pub fn new_local(