Add feature flag related issues to registration features

What does this MR do and why?

We want to add Feature flag related issues to Registration features available to users who have service ping data sharing enabled.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Testing previous behavior:
   • Make sure you're on a Premium/Ultimate plan GitLab GDK instance
   • Select some project
   • Select Deploy -> Feature flags in sidebar.
   • Create new feature flag
   • Edit created flag and check you can see Linked items section.
   • Add some issue to Linked items and save.
   • Open that linked issue and verify you can see Related feature flags section.
2. When registration features are enabled:
   • Make sure you're on a free plan GitLab GDK instance (for example, remove current license or stub #current to return nil)
   • The check_namespace_plan setting or the should_check_namespace_plan? method should be set to false
   • Make sure you have the Registration Features checkbox enabled (Admin -> Settings -> Metrics and profiling -> Usage statistics -> Enable Service Ping & Enable Registration Features)
   • Select some project
   • Select Deploy -> Feature flags in sidebar.
   • Create new feature flag
   • Edit created flag and check you can see Linked items section.
   • Add some issue to Linked items and save.
   • Open that linked issue and verify you can see Related feature flags section.
3. When registration features are disabled:
   • Make sure you're on a free plan GitLab GDK instance
   • The check_namespace_plan setting or the should_check_namespace_plan? method should be set to false
   • Make sure you have the registration features checkbox disabled (Admin -> Settings -> Metrics and profiling -> Usage statistics -> Enable Registration Features)
   • Select some project
   • Select Deploy -> Feature flags in sidebar.
   • Create new feature flag
   • Edit created flag and check you don't see Linked items section.
4. Make sure that the new text appears on the docs page:
   • run gdk restart gitlab-docs
   • go to <local_gitlab_docs_host>/ee/user/admin_area/settings/usage_statistics.html#registration-features-program and make sure the new section is on the page and the link works

Updated doc	Link page

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 #391540 (closed)