feat(css): Add focus variant for gl-z-index-1

mentioned in commit 583b1ae3

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!227 (merged)

mentioned in merge request gitlab!135848 (merged)

mentioned in merge request gitlab-docs!4372 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3598 (merged)

mentioned in merge request status-page!886 (merged)

mentioned in merge request status-page!888 (merged)