feat(css): add gl-sm-mt-6

mentioned in commit 5e1a2e3e

mentioned in merge request status-page!604 (merged)

mentioned in merge request gitlab!97530 (merged)

mentioned in merge request gitlab-docs!3109 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3059 (merged)

mentioned in merge request gitlab-docs!3125 (merged)

mentioned in merge request status-page!609 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!87 (merged)