feat(Tailwind): Add subtle and strong border tokens to shadow utils

mentioned in commit 8e483fc6

mentioned in merge request status-page!903

mentioned in commit 49144004

mentioned in merge request gitlab!178190 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4259 (merged)

mentioned in merge request gitlab!180089 (closed)

mentioned in merge request gitlab!180099 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1485 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!64 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!367 (merged)