fix(dropdown): set aria-expanded to string

mentioned in commit 792634f1

mentioned in merge request status-page!893 (merged)

mentioned in merge request gitlab!145161 (merged)

mentioned in merge request gitlab!145335 (merged)

mentioned in merge request gitlab!145358 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3755 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3756 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3757 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3758 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!26 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!300 (merged)