feat(Borders): Update subtle dark

mentioned in commit 990df1f1

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1318 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!173815 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!356 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!357 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4193 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4194 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!57 (merged)