fix(GlDropdown): Add "link" variant as valid option for dropdowns

mentioned in commit af2f0962

mentioned in merge request rpadovani/daintree!307 (merged)

mentioned in merge request gitlab!45607 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2125 (merged)

mentioned in merge request status-page!129 (merged)

mentioned in merge request gitlab!45765 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2128 (merged)

mentioned in merge request status-page!130 (merged)

mentioned in merge request gitlab-docs!1270 (merged)