Fix datetime

gateway/app/core/service_discovery.py

@@ -96,7 +96,7 @@ class ServiceDiscovery:
             key = f"service_health:{service_name}"
             value = {
                 "healthy": is_healthy,
-                "last_check": datetime.utcnow().isoformat(),
+                "last_check":  datetime.now(datetime.timezone.utc).isoformat(),
                 "url": self.services[service_name]
             }