Stable stein is in Extended maintenance state[1] and
As per Tempest stable branmch testing policy, it stopped
the support for stable/stein in master version. Tempest 26.0.0
tag is last supported tag for stein.
Use that tag to test stable/stein branch before any incompatible
change merge in Tempest master.
[1] https://releases.openstack.org/
Change-Id: I715638f11d622de12439cb97a4be999cfab40367