End support for stable/xena

This marks the end of support for stable/xena
which is EM now [1]. This remove the stable/xena
jobs and also adds relesenote to mark the end of
support of Xena.

[1] https://review.opendev.org/c/openstack/releases/+/881254
    https://releases.openstack.org/

Change-Id: I1027823aa378e9c669dca5a1bc49a9ada604a3d7
3 files changed