Move the context switcher to a disclosure dropdown

What does this MR do and why?

This drops the GlCollapse-based context switcher in favor of a GlDisclosureDropdown solution. This aligns with most recent design decisions to make the context switcher stand out more.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the new nav:

   1. Enable the feature flag:

      echo "Feature.enable(:super_sidebar_nav)" | rails c

   2. Enable the user setting 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.