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/markdown/src/markdown.rs

@@ -821,13 +821,13 @@ impl Element for MarkdownElement {
         let mut context = KeyContext::default();
         context.add("Markdown");
         window.set_key_context(context);
-        let model = self.markdown.clone();
+        let entity = self.markdown.clone();
         window.on_action(std::any::TypeId::of::<crate::Copy>(), {
             let text = rendered_markdown.text.clone();
             move |_, phase, window, cx| {
                 let text = text.clone();
                 if phase == DispatchPhase::Bubble {
-                    model.update(cx, move |this, cx| this.copy(&text, window, cx))
+                    entity.update(cx, move |this, cx| this.copy(&text, window, cx))
                 }
             }
         });