Update grenade setting in stable/2024.1

Along with updating the grenade setting for
stable/2024.1, adding the forward upgrade job as experimental.

Change-Id: I5009c790f86c0c89bb95e42a91835f36909fd69b

.zuul.yaml

@@ -19,6 +19,7 @@
         - grenade-skip-level
     experimental:
       jobs:
+        - grenade-forward
         - grenade-postgresql
 
 - job:
@@ -346,7 +347,8 @@
     vars:
       # When a stable branch stable/X is created from master,
       # this value should be updated to stable/X.
-      grenade_from_branch: master
+      grenade_from_branch: stable/2024.1
+      grenade_to_branch: master
 
 - job:
     name: grenade-dsvm-neutron-forward
@@ -415,5 +417,3 @@
       # release only.
     vars:
       grenade_from_branch: stable/2023.1
-    branches:
-      - master

roles/configure-grenade-branches/defaults/main.yaml

@@ -1,3 +1,3 @@
 ---
 grenade_from_branch: stable/2023.2
-grenade_to_branch: master
+grenade_to_branch: stable/2024.1