Simplify structure of CI/CD variables page

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/108887



Merged-by: Andrew Fontaine <afontaine@gitlab.com>
Approved-by: Diana Logan <dlogan@gitlab.com>
Approved-by: Andrew Fontaine <afontaine@gitlab.com>
Co-authored-by: Marcel Amirault <mamirault@gitlab.com>