feat(css): Add .gl-flex-basis-two-thirds

mentioned in commit 10b8afb3

mentioned in merge request status-page!612 (merged)

mentioned in merge request gitlab-docs!3146 (merged)

mentioned in merge request gitlab!100019 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3070 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!87 (merged)