fix: replace colocate_ref_and_trainer with colocate list in qwen3_8b yaml

The old colocate_ref_and_trainer bool was replaced by the generalized
colocate list field in LauncherConfig. Update the yaml to match.

Author: joecummings

apps/grpo/qwen3_8b.yaml

@@ -16,7 +16,7 @@ rollout_threads: ${services.generator.num_replicas}   # Recommended to set equal
 provisioner:
   launcher: ssh
   ssh_hostfile: "/job/hostfile"
-  colocate_ref_and_trainer: true
+  colocate: [ref_model, trainer]
 
 # Observability configuration
 metric_logging: