auth: Fix logic in `service::role_has_superuser`
auth: Fix logic in `service::role_has_superuser`

The previous code has an off-by-one error since the iterator is
incremented unconditionally prior to being compared to the end of the
collection.

This new version is also shorter thanks to `seastar::do_until`.

scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla