Add link to GitLab Next to new User menu

What does this MR do and why?

Resolves #390390 (closed) by adding the "Switch to GitLab Next" link to the new User menu (behind the super_sidebar_nav feature flag).

Screenshots or screen recordings

How to set up and validate locally

This MR uses existing helpers, but it's still worth both scenarios:

SaaS mode

• Make sure your GDK is configure to simulate a SaaS instance.
• Enable the super_sidebar_nav feature flag and toggle the "New navigation" to on in the user dropdown (on the top-right on the "old" navigation).
• Looking at the new navigation, open the new user menu by clicking your avatar in the lefthand sidebar.
• You should see a "Switch to GitLab Next" link.
• Clicking it should bring you to next.gitlab.com

Self-managed mode

• Make sure your GDK is configure to not simulate a SaaS instance.
• Check the new user menu as described above. The GitLab Next link should not be visible.

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 #385749 (closed)