Add Service Level Agreement countdown timer to registration features

What does this MR do and why?

We want to add Service Level Agreement countdown timer to 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 Settings -> Monitor -> Incidents -> Incident settings tab
   • Select Activate “time to SLA” countdown timer
   • Set a time limit and save
   • Create new Incident & check you can see SLA section on:
     • Incident show page
     • Incidents list page (SLA column)
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)
   • Make sure you have the Registration Features checkbox enabled (Admin -> Settings -> Metrics and profiling -> Usage statistics -> Enable Service Ping & Enable Registration Features)
   • Follow above steps from 1. to verify it.
3. When registration features are disabled:
   • Make sure you're on a free plan GitLab GDK instance
   • Make sure you have the registration features checkbox disabled (Admin -> Settings -> Metrics and profiling -> Usage statistics -> Enable Registration Features)
   • Select some project
   • Feature should be disabled and not usable
     • No Incident settings tab in Settings -> Monitor -> Incidents
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 #422269 (closed)