feat(css): Added gl-xs-mb-4 utility classes

mentioned in commit 74c572a8

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2690 (closed)

mentioned in merge request status-page!447 (merged)

mentioned in merge request gitlab!78336 (merged)

mentioned in merge request gitlab-docs!2365 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2698 (merged)