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

See merge request gitlab-org/gitlab-ui!4939



Merged-by: Scott de Jonge <sdejonge@gitlab.com>
Approved-by: Scott de Jonge <sdejonge@gitlab.com>
Co-authored-by: Florie Guibert <fguibert@gitlab.com>

# [107.4.0](gitlab-org/gitlab-ui@v107.3.0...v107.4.0) (2025-01-21)

### Features

* **Tailwind:** Add subtle and strong border tokens to shadow utils ([3584dfc6](gitlab-org/gitlab-ui@3584dfc6))