feat(GlBroadcastMessage): Add hardcoded sr-only heading

mentioned in commit 20cf48c9

mentioned in merge request status-page!780 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!142 (merged)

mentioned in merge request gitlab!119812 (merged)

mentioned in merge request gitlab-docs!3815 (merged)

mentioned in merge request gitlab!119966 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3394 (merged)