Update CI to use unversioned jobs template

As part of the migration of this project to the independent release
model, we failed to notice that the job template was still tied to a
specific release. We've now introduced a new unversioned job template,
'openstack-python3-jobs' [1], which was can and should use. Do this.

[1] https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/

Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/
Change-Id: I2ae92713a304ba24157fe0b8580bed691f3811e5
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.zuul.yaml

@@ -3,7 +3,7 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3