Switch testing to Xena testing runtime

Upating the tetsing template to Xena testing runtime:
https://governance.openstack.org/tc/reference/runtimes/xena.html

Change-Id: Ic0a086d06aa78280cc29a893c68c4dceb73dd1a3

.zuul.yaml

@@ -1,6 +1,6 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-python3-victoria-jobs
+      - openstack-python3-xena-jobs
       - release-notes-jobs-python3
       - publish-openstack-docs-pti