[Kolla] Transition Xena to EOL

Hello,

As aggreed on IRC meeting, Kolla Xena deliverables (Kolla, Kolla-Ansible, Kayobe, Ansible-Collection-Kolla) are going EOL. [1] Yoga, Zed and Antelope are our current stable releases. Xena was in Extended Maintenance for a some time, Kolla community does not have resources to support it actively. All changes for Kolla project deliverables in Xena branches have been abandoned.

[1] https://review.opendev.org/c/openstack/releases/+/894630

Best regards, Bartosz Bezak