feat(GlMarkdown): Use heading scale

mentioned in commit 646b1e2a

mentioned in merge request status-page!893 (merged)

mentioned in merge request gitlab!145358 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3756 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3757 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!26 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!300 (merged)