feat(css): added gl-z-index-999 utility

mentioned in commit 83b826b4

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!221 (merged)

mentioned in merge request gitlab!134583 (merged)

mentioned in merge request gitlab-docs!4337 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3578 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!225 (closed)

mentioned in merge request status-page!881 (merged)

mentioned in merge request status-page!883 (merged)