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/repl/src/notebook/cell.rs

@@ -127,7 +127,7 @@ impl Cell {
             } => {
                 let source = source.join("");
 
-                let model = cx.new(|cx| {
+                let entity = cx.new(|cx| {
                     let markdown_parsing_task = {
                         let languages = languages.clone();
                         let source = source.clone();
@@ -159,7 +159,7 @@ impl Cell {
                     }
                 });
 
-                Cell::Markdown(model)
+                Cell::Markdown(entity)
             }
             nbformat::v4::Cell::Code {
                 id,