Forced colors mode: Review use of box-shadow as border

We use box-shadow as border in some GitLab UI components. This leads to a problem where it gets ignored in forced colors mode and elements appear with no clear separation. A UX Paper Cuts in GitLab.org / gitlab-ui fix was introduced in High contrast mode: Improve readability of comp... (gitlab-org/gitlab-ui!4146 - merged) but we should have a more holistic view on this problem to avoid it in the first place.

We might want to also review our product for the use of box-shadow as borders.

Follow-up to gitlab-org/gitlab-ui!4146 (merged), gitlab-org/gitlab!152067 (merged)