feat(css): adds gl-md-flex-basis-13 utility class

mentioned in commit e16f59fe

mentioned in merge request gitlab!144134 (merged)

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

mentioned in merge request gitlab!144286 (merged)

mentioned in merge request gitlab-docs!4595 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3734 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!26 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!300 (merged)