Unmaintained-Only: change stable/yoga to unmaintained

Grenade skip level jobs reference stable/yoga but that branch
was renamed to unmaintained/yoga

Change-Id: Ifdfa194a95a39c63ec61f2afe030fed108de5a77
Related-Bug: #2052915

Author: elajkat

zuul.d/grenade.yaml

@@ -374,7 +374,7 @@
       # Move this forward when master changes to a new skip-level-allowed
       # target release. Right now, this is xena because master is zed.
       # When master is A, this should become yoga, and so forth.
-      grenade_from_branch: stable/yoga
+      grenade_from_branch: unmaintained/yoga
       grenade_localrc:
         NOVA_ENABLE_UPGRADE_WORKAROUND: True
 
@@ -388,6 +388,6 @@
       # Move this forward when master changes to a new skip-level-allowed
       # target release. Right now, this is xena because master is zed.
       # When master is A, this should become yoga, and so forth.
-      grenade_from_branch: stable/yoga
+      grenade_from_branch: unmaintained/yoga
       grenade_localrc:
         NOVA_ENABLE_UPGRADE_WORKAROUND: True