New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/auto_update_ui/src/auto_update_ui.rs

@@ -93,7 +93,7 @@ fn view_release_notes_locally(
 
                             let tab_description = SharedString::from(body.title.to_string());
                             let editor = cx.new(|cx| {
-                                Editor::for_multibuffer(buffer, Some(project), true, window, cx)
+                                Editor::for_multibuffer(buffer, Some(project), window, cx)
                             });
                             let workspace_handle = workspace.weak_handle();
                             let markdown_preview: Entity<MarkdownPreviewView> =