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/image_store.rs

@@ -387,18 +387,18 @@ impl ImageStoreImpl for Entity<LocalImageStore> {
             let LoadedBinaryFile { file, content } = load_file.await?;
             let image = create_gpui_image(content)?;
 
-            let model = cx.new(|cx| ImageItem {
+            let entity = cx.new(|cx| ImageItem {
                 id: cx.entity_id().as_non_zero_u64().into(),
                 file: file.clone(),
                 image,
                 reload_task: None,
             })?;
 
-            let image_id = cx.read_entity(&model, |model, _| model.id)?;
+            let image_id = cx.read_entity(&entity, |model, _| model.id)?;
 
             this.update(&mut cx, |this, cx| {
                 image_store.update(cx, |image_store, cx| {
-                    image_store.add_image(model.clone(), cx)
+                    image_store.add_image(entity.clone(), cx)
                 })??;
                 this.local_image_ids_by_path.insert(
                     ProjectPath {
@@ -415,7 +415,7 @@ impl ImageStoreImpl for Entity<LocalImageStore> {
                 anyhow::Ok(())
             })??;
 
-            Ok(model)
+            Ok(entity)
         })
     }