[all][tc] Dropping lower-constraints testing from all projects

Hello Everyone, You might have seen the discussion around dropping the lower constraints testing as it becomes more challenging than the current value of doing it. Few of the ML thread around this discussion: - http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019521.... - http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.... As Oslo and many other project dropping or already dropped it, we should decide it for all other projects also otherwise it can be more changing than it is currently. We have not defined it in PTI or testing runtime so it is always up to projects if they still want to keep it but we should decide a general recommendation here. -gmann