Add EOM tag option to transition_series.sh

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

tools/transition_series.sh

@@ -10,7 +10,7 @@ BASEDIR=$(dirname $TOOLSDIR)
 if [[ $# -lt 2 ]]; then
     echo "Usage: $(basename $0) <series> <tag>"
     echo ""
-    echo "<tag> either 'em' or 'eol'"
+    echo "<tag> either 'em', 'eom' or 'eol'"
     exit 1
 fi