Use stable constraints for Tempest venv for stable/stein testing

As stable/stein is in Extended Maintenance state, we use old Tempest
26.0.0 for its testing
- https://review.opendev.org/c/openstack/devstack/+/774103

For old Tempest, we need to use the stable constraints otherwise
it recreate the tempest venv and remove the initial installed tempest
plugins which causes the error while running plugins tests

Example: https://zuul.opendev.org/t/openstack/build/bae42db334ca4b69ab3f4b1c440bcb6a/log/job-output.txt#2444-2454

For Devstack side, I am making the constraints setting more
standardize in below series but for legacy job we need the fix here.
- https://review.opendev.org/q/topic:%22standardize-tempest-tox-constraints%22+(status:open%20OR%20status:merged)

Depends-On: https://review.opendev.org/c/openstack/devstack/+/776811
Depends-On: https://review.opendev.org/c/openstack/tempest/+/774060

Change-Id: I36799ee9d4db20104377e2772c86229b8083448d

devstack-vm-gate.sh

@@ -864,7 +864,7 @@ if [[ "$DEVSTACK_GATE_TEMPEST" -eq "1" ]]; then
     # instead of using master. We need to export it via env var TOX_CONSTRAINTS_FILE
     # so that initial creation of tempest tox use stable branch constraint
     # instead of master constraint as defined in tempest/tox.ini
-    stable_for_u_c="stable/[o-r]"
+    stable_for_u_c="stable/[o-s]"
     if [[ "$ZUUL_BRANCH" =~ $stable_for_u_c  ]] ; then
         export TOX_CONSTRAINTS_FILE=$BASE/new/requirements/upper-constraints.txt
     else