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/task_store.rs

@@ -51,7 +51,7 @@ impl EventEmitter<crate::Event> for TaskStore {}
 impl TaskStore {
     pub fn init(client: Option<&AnyProtoClient>) {
         if let Some(client) = client {
-            client.add_model_request_handler(Self::handle_task_context_for_location);
+            client.add_entity_request_handler(Self::handle_task_context_for_location);
         }
     }