fix(DesignTokens): use gray-950 for dark color contrast background

mentioned in commit 8ef1e791

mentioned in merge request status-page!893 (merged)

mentioned in merge request gitlab!141558 (merged)

mentioned in merge request gitlab!141583 (merged)

mentioned in merge request gitlab-docs!4525 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3691 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!268 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!269 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!21 (merged)