Migrate workspace variables card to CrudComponent

What does this MR do and why?

Migrate workspace variables card to CrudComponent (see #476988 for more details)

Screenshots or screen recordings

Before	After

How to set up and validate locally

http://127.0.0.1:3000/-/remote_development/workspaces/new

Links

• ✂️ UX Paper Cuts 17.3 → Dark mode - Design Support (#452231 - closed)
• Migration: GlCard (gl-new-card variant) to Crud (#476988)