feat(GlDropdown): Replace BootstrapCSS Util Class with GitLab UI Class

mentioned in commit 7c839e76

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!171760 (closed)

mentioned in commit e9c7a9b7

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4168 (merged)

mentioned in merge request gitlab!172328 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1249 (closed)

mentioned in commit e2ff97db

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)