Update ListboxHelper to adhere to Pajamas style

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What does this MR do and why?

Fixes: #417800 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

See !151151 (comment 1911124165).

How to set up and validate locally

1. Visit some of the places this helper is used:
   • project forks page http://gdk.test:3000/gitlab-org/gitlab-test/-/forks
   • project starrers page http://gdk.test:3000/gitlab-org/gitlab-test/-/starrers
2. Ensure the dropdown looks correct. Ideally, load the page with JavaScript disabled, since the markup this changes is completely replaced by Vue components once JS is executed.
3. Bonus points: compare DOM structure with a real listbox (just the button and parent element, not the menu).