fix(GlBaseDropdown): use aria-labelledby properly

mentioned in commit c43fb9b2

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!373 (merged)

mentioned in merge request gitlab-svgs!1155 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!70 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!190119 (merged)

mentioned in commit fbf7991c

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4407 (merged)

mentioned in merge request gitlab-org/technical-writing/docs-gitlab-com!683 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1731 (merged)