fix(system-upgrades): Restrict fedora version upgrades

This patch restrict Fedora upgrades to nodes with an explicit label
`upgrade.shivering-isles.com/fedora-35-to-36=true`. This ensures a
controlled transition between versions.

infrastructure/system-upgrades/fedora-system-upgrade.yaml

@@ -35,6 +35,10 @@ spec:
         values:
           - "35"
           - "36"
+      - key: upgrade.shivering-isles.com/fedora-35-to-36
+        operator: In
+        values:
+          - "true"
   serviceAccountName: system-upgrade
   secrets:
     - name: fedora-35-to-36-system-upgrade