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/recent_projects/src/remote_servers.rs

@@ -1263,7 +1263,7 @@ impl RemoteServerProjects {
                 state = new_state.clone();
             }
         }
-        let scroll_state = state.scrollbar.parent_model(&cx.entity());
+        let scroll_state = state.scrollbar.parent_entity(&cx.entity());
         let connect_button = div()
             .id("ssh-connect-new-server-container")
             .track_focus(&state.add_new_server.focus_handle)