feat(Banner): Update heading level

mentioned in commit 497b5d45

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

mentioned in merge request gitlab!97530 (merged)

mentioned in merge request gitlab-docs!3092 (merged)

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

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

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