Remove stable/stein testing jobs

stable/stein is not suported in Patrole now, so
let's remove their jobs from master gate.

In order to pass the CI, the following changes are also made:
* Added skip for Nova policy
  "os_compute_api:os-admin-actions:reset_network", which was removed in
  https://review.opendev.org/c/openstack/nova/+/749315
* Removed openstack-tox-lower-constraints job for now until we have a
  solution.

Change-Id: Id73342c24342637edc37104f2112235a2edcac39
2 files changed