CI: Fix unmaintained/2023.1 branch

Antelope is now called `unmaintained/2023.1`, not `stable/2023.1`. This
fix is requred for unmaintained/2023.1 CI in Cinder.

This is adapted from If769d9da92b724d40f47c9a57a75c021809c8850 which was
fixing the same issue for Zed.

NOTE(elod.illes): cinderlib is retired, hence the repository is empty on
master branch, causing issues with build-openstack-releasenotes job,
which should run on master. To unblock the gate this job (actually,
release-notes-jobs-python3 job template) needs to be dropped.

Change-Id: I04a9b621c12892ac1b0f53d97f43f38df160e7a0

.zuul.yaml

@@ -4,7 +4,6 @@
     queue: integrated
     templates:
       - publish-openstack-docs-pti
-      - release-notes-jobs-python3
     check:
       jobs:
         - openstack-tox-pep8

tox.ini

@@ -36,8 +36,8 @@ usedevelop=True
 # NOTE: Functional tests may fail if host is missing bindeps from deps projects
 deps =
     -r{toxinidir}/test-requirements.txt
-    git+https://opendev.org/openstack/os-brick@stable/2023.1
-    git+https://opendev.org/openstack/cinder@stable/2023.1
+    git+https://opendev.org/openstack/os-brick@unmaintained/2023.1
+    git+https://opendev.org/openstack/cinder@unmaintained/2023.1
 
 commands =
     find . -ignore_readdir_race -type f -name "*.pyc" -delete