fix(GlEmptyState): update styling to be Pajamas compliant

mentioned in commit 3dd97a3b

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!204 (merged)

mentioned in merge request status-page!872 (merged)

mentioned in merge request gitlab!131146 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3542 (merged)

mentioned in merge request gitlab-docs!4256 (merged)

mentioned in merge request gitlab-docs!4258 (closed)