Super sidebar navigation item with children read to screen-readers

What does this MR do and why?

• Use <button> for section nav item with aria-expanded attribute
• Remove aria-hidden
• Add aria-controls to link <button> to <ul>
• Add aria-label to <ul> to read "Plan list X items" in NVDA/JAWS

Screenshots or screen recordings

Before	After
CleanShot_2023-03-09_at_16.46.45	CleanShot_2023-03-09_at_16.45.30
CleanShot_2023-03-10_at_14.21.21	CleanShot_2023-03-10_at_14.19.51

How to set up and validate locally

• Enable the feature flag in rails console with rails c

Feature.enable(:super_sidebar_nav)

• Enable the "New navigation" setting toggle from the user dropdown

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #394973 (closed)