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/outline_panel/src/outline_panel.rs

@@ -796,9 +796,9 @@ impl OutlinePanel {
                 show_scrollbar: !Self::should_autohide_scrollbar(cx),
                 hide_scrollbar_task: None,
                 vertical_scrollbar_state: ScrollbarState::new(scroll_handle.clone())
-                    .parent_model(&cx.entity()),
+                    .parent_entity(&cx.entity()),
                 horizontal_scrollbar_state: ScrollbarState::new(scroll_handle.clone())
-                    .parent_model(&cx.entity()),
+                    .parent_entity(&cx.entity()),
                 max_width_item_index: None,
                 scroll_handle,
                 focus_handle,