[horizon] Transition EM branches to Unmaintained

With the new TC resolution the community replaces Extended Maintenance
with Unmaintained status [1].

So this patch transitions victoria, wallaby, and xena EM branches to
Unmaintained as follows:
- tip of the given branch will be tagged with <series>-eom tag
- unmaintained/<series> branch will be cut from <series>-eom tag
- stable/<series> branch will be deleted

Some further notes:
- stable policy still applies
- backports can be pushed against unmaintained/<series> branch
- openstack-unmaintained-core group have the rights to merge patches

For further details please read the resolution [1] & review and +1 this
patch if this is correct as soon as possible.

[1] https://governance.openstack.org/tc/resolutions/20230724-unmaintained-branches.html

Change-Id: I1f7f44b05b77885149946f0ca251a2c6cf8b82f6

deliverables/victoria/horizon.yaml

@@ -42,9 +42,15 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: c9ebeb1fa3de5ead102fef2027c8e5320c3038e1
+  - version: victoria-eom
+    projects:
+      - repo: openstack/horizon
+        hash: 7d3e351d8dba985acd1b11da9775b4c7b7b7dff7
 branches:
   - name: stable/victoria
     location: 18.6.0
+  - name: unmaintained/victoria
+    location: victoria-eom
 cycle-highlights:
   - Error messages shown in horizon now contains more detail. Previously
     GUI users cannot know detail reasons of operations, but users can

deliverables/wallaby/horizon.yaml

@@ -39,7 +39,13 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: beed6bf6f6f83df9972db5fb539d64175ce12ce9
+  - version: wallaby-eom
+    projects:
+      - repo: openstack/horizon
+        hash: 9b1a13e0837b9f0ac3e0e919ad0a270d417003db
 branches:
   - name: stable/wallaby
     location: 19.1.0
+  - name: unmaintained/wallaby
+    location: wallaby-eom
 release-notes: https://docs.openstack.org/releasenotes/horizon/wallaby.html

deliverables/xena/horizon.yaml

@@ -38,7 +38,13 @@ releases:
     projects:
       - repo: openstack/horizon
         hash: 07343c853af4e9b74fe11faaa2bbc1a8c1e62b78
+  - version: xena-eom
+    projects:
+      - repo: openstack/horizon
+        hash: 050914d8b68bf049097bdd5656084df9ba64e684
 branches:
   - name: stable/xena
     location: 20.1.0
+  - name: unmaintained/xena
+    location: xena-eom
 release-notes: https://docs.openstack.org/releasenotes/horizon/xena.html