Stub feature flag super_sidebar_nav_enrolled

What does this MR do and why?

We have received an alert from Upstream:

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126175#note_1534100298

The merger of this Upstream MR will break JH's pipeline. JH can prepare in advance for this Upstream MR changes.

Screenshots or screen recordings

Non-UI

How to set up and validate locally

• Create a new branch on jh, cherry pick commits of the upstream MR;
• Fix failed test cases;
• Revert commits of the upstream MR;

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.