[MISC] Pin grafana-agent-k8s channel to 1/stable (#624)

Author: sinclert-canonical

tests/integration/bundle_templates/grafana_agent_integration.j2

@@ -10,7 +10,7 @@ applications:
       mysql-image: {{ mysql_image_source }}
   grafana-agent-k8s:
     charm: grafana-agent-k8s
-    channel: latest/stable
+    channel: 1/stable
     scale: 1
 relations:
   - [mysql-k8s:metrics-endpoint, grafana-agent-k8s:metrics-endpoint]