Fix double border in closed widget wrapper

Summary

The Child items box on issues has a double border in its closed state.

Steps to reproduce

1. Go to an issue
2. Close the Child items box with the chevron on the right

Relevant logs and/or screenshots

Current	Expected

Implementation Guide

The component responsible for collapsing the Child items is the widget wrapper, found at app/assets/javascripts/work_items/components/widget_wrapper.vue. We should:

1. Add :aria-expanded="isOpen.toString()" to the top-level gl-new-card in the widget wrapper.
   For reference, aria-expanded=false is used in GitLab UI to hide the superfluous bottom border if the card is collapsed:

   .gl-new-card[aria-expanded=false] .gl-new-card-header {
     border-bottom: 0;
     border-radius: .25rem
   }

2. Follow the Steps to reproduce above to see if the fix worked
3. We can optionally add a test in spec/frontend/work_items/components/widget_wrapper_spec.js to see that aria-expanded is false if collapsed, and true if not collapsed. You can add a data-testid to the gl-new-card (where you added the aria-expanded) and use the wrapper.findByTestId method to access the element like the widget-body.