With the new TC resolution the community replaces Extended Maintenance
with Unmaintained status [1].
This patch introduces the new <series>-eom tag option for the
new-release command. The tag applies at the tip of the given
stable/<series> branch and the new unmaintained/<series> branch can be
cut from that tag.
[1] https://governance.openstack.org/tc/resolutions/20230724-unmaintained-branches.html
Change-Id: Ie67da6f4857f0d438733799bf40f8ece66f67d32