fix(Typography): remove scale from gl-heading-5 and gl-heading-6 classes

mentioned in commit a00c6e2c

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

mentioned in merge request gitlab-docs!4555 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3713 (merged)

mentioned in merge request gitlab!143271 (merged)

mentioned in merge request gitlab!143284 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!268 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!269 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!21 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3722 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3723 (merged)