feat(GlModal): unbox component by removing borders

mentioned in commit 5b751a15

mentioned in merge request gitlab!79962 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2710 (closed)

mentioned in merge request status-page!459 (merged)

mentioned in merge request gitlab-docs!2416 (merged)

mentioned in merge request gitlab!80054 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2714 (merged)