feat(tailwind): add transparent to supported colors

mentioned in commit a68774f7

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!312 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!28 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!149341 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3814 (merged)