With the new TC resolution the community replaces Extended Maintenance
with Unmaintained status [1].
This patch introduces the new <series>-eom tag option in the
transition_series.sh script. 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: Ia7064e36a2d92ef02f60b7371ebf3e4772abcc80