With the new TC resolution the community replaces Extended Maintenance
with Unmaintained status [1]. To accomodate this, validator needs to be
extended to accept <series>-eom tags that applies at the tip of the
given stable/<series> branch and the new unmaintained/<series> branch
will be cut from that tag.
[1] https://governance.openstack.org/tc/resolutions/20230724-unmaintained-branches.html
Change-Id: I606ba5e5f8fd783d6c3f83356ded27a46e8bac56