Add quality_scale to airnow manifest

Author: HoffmanEl

homeassistant/components/airnow/manifest.json

@@ -7,5 +7,6 @@
   "integration_type": "service",
   "iot_class": "cloud_polling",
   "loggers": ["pyairnow"],
+  "quality_scale": "legacy",
   "requirements": ["pyairnow==1.3.1"]
-}
+}