With the new unmaintained process [1], branches are being tagged with
EOM before being removed.
This behaviour breaks reno, making it struggle to find respective origin
and leads to releasenotes job failure.
In order to overcome the issue we add eom tag to the list of expected ones
when we can not find the refferenced branch.
With that, EOL tag will have prescedence over EOM one.
[1] https://governance.openstack.org/tc/resolutions/20230724-unmaintained-branches.html
Change-Id: Ic641af5aa65add9751b49fb47691a98f80a7c5a8
47651ec906