Fix grenade-skip-level job to use unmaintained/2023.1

Projects are transitioning to Unmaintained and their stable/2023.1
branches were deleted and unmaintained/2023.1 were cut instead, so we
have to use that instead.

Since testing unmaintained things could not be as a requirement, the
patch set the job as non-voting on the check queue and removes from the
gate queue to reduce CI resource usage.

Change-Id: I1ba3fb368e523d31156f45d2e2ef28ba22dfdb91

.zuul.yaml

@@ -9,14 +9,14 @@
         - octavia-grenade
         - ironic-grenade:
             voting: false
-        - grenade-skip-level
+        - grenade-skip-level:
+            voting: false
     gate:
       jobs:
         - grenade
         - grenade-multinode
         - grenade-heat-multinode
         - octavia-grenade
-        - grenade-skip-level
     experimental:
       jobs:
         - grenade-forward
@@ -395,7 +395,7 @@
       # For the base "always" job, this should always be the N-2 release from
       # current master for projects that want to assert N-2 upgradability
       # on all releases.
-      grenade_from_branch: stable/2023.1
+      grenade_from_branch: unmaintained/2023.1
       grenade_localrc:
         NOVA_ENABLE_UPGRADE_WORKAROUND: True
 
@@ -416,4 +416,4 @@
       # also and in this job definition, we control for everyone to run on SLURP
       # release only.
     vars:
-      grenade_from_branch: stable/2023.1
+      grenade_from_branch: unmaintained/2023.1