feat(css): Added gl-opacity-6 & gl-opacity-7 utility classes

mentioned in commit 19518e87

mentioned in merge request gitlab!78336 (merged)

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

mentioned in merge request status-page!448 (merged)

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

mentioned in merge request gitlab-docs!2384 (merged)

mentioned in merge request gitlab-docs!2385 (merged)