When User cap is enabled we mark all newly created users as pending
approval, even those that are not humans. This prevents creation of
project access tokens - at the time when we try to create the token the
associated bot users is not approved.

This updates `Users::BuildService#set_pending_approval_state` to mark
users as pending approval only if they are `human?`.

See https://gitlab.com/gitlab-org/gitlab/-/issues/323191.