feat: Add border-purple-300 to mixins

mentioned in commit ac99fb20

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!168 (merged)

mentioned in merge request status-page!830 (merged)

mentioned in merge request gitlab!126331 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3432 (merged)

mentioned in merge request gitlab-docs!4074 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3479 (merged)