Remove testing with stable/zed

... because stable/zed is now unmaintained. Note that stable/2024.1
supports Puppet 6 but we already removed any tests with Puppet 6 during
that cycle so no additional stable branch job is added.

Change-Id: Ia5b8e84653cf7ed581a21b8b369a9fd214a8aac0

.zuul.yaml

@@ -5,19 +5,3 @@
       - puppet-openstack-integration-jobs-scenario001
       - puppet-openstack-integration-jobs-scenario004
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - puppet-ceph-unit-6.28-ubuntu-focal-zed
-    gate:
-      jobs:
-        - puppet-ceph-unit-6.28-ubuntu-focal-zed
-
-# NOTE(tkajinam): stable/zed supports Ubuntu 20.04 and Puppet 6
-- job:
-    name: puppet-ceph-unit-6.28-ubuntu-focal-zed
-    parent: puppet-openstack-unit-base
-    nodeset: ubuntu-focal
-    override-checkout: stable/zed
-    vars:
-      puppet_gem_version: 6.28.0
-      rspec_puppet_version: '= 2.9.0'