Fix supported releases and py versions in the doc

Our documentation showed that Victoria, Ussuri and Train are the
supported releases by tempest's master, which wasn't the case.
It also showed that tempest is compatible with python 3.6, 3.7
and 3.8. We dropped py 3.6 and 3.7 support by [1].

[1] https://review.opendev.org/c/openstack/tempest/+/843182

Closes-Bug: #1976282
Change-Id: I0ccfa824f08cbdf58f26a142dac3a0881605ef08
1 file changed