The stable/xena branch has been deleted, so reno can't find its
release notes. Use the xena-eom tag to indicate the end of the
Glance project's maintenance of the Xena series.
This strategy is consistent with the way we handled stable/yoga and
was agreed upon at the 8 Feb Glance meeting:
https://meetings.opendev.org/meetings/glance/2024/glance.2024-02-08-14.00.log.html#l-58
Change-Id: I3473a36a473767531c19463611d3eddb1a676998