Add GitLab-Pages version to Admin Dashboard

What does this MR do?

Displays the GitLab-Pages version in the admin dashboard.

Are there points in the code the reviewer needs to double check?

• Is pulling the version number from the GITLAB_PAGES_VERSION file the correct place?
• Should the GitLab-Pages version not be listed if GitLab-Pages is disabled?

Why was this MR needed?

Requested in #36953 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #36953 (closed)