chore: release bigquery 1.21.0 (#4614)
chore: release bigquery 1.21.0 (#4614)

Co-authored-by: shollyman <10194de3bbef2536f4b5d02c780e8788d90cab2e@google.com>
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go