Tidy up js_example

Problem/Motivation

The "core/jquery.ui.accordion" asset library is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. See https://www.drupal.org/node/3067969

Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 when using an install profile that does not set a default theme. See https://www.drupal.org/node/3083055, which includes recommendations on which theme to use.

Proposed resolution

Use https://www.drupal.org/project/jquery_ui_accordion instead