Replace hardcoded icon with 'class: duration' for runtime

custom_components/tuya_local/devices/drexma_et7aw_thermostat.yaml

@@ -288,7 +288,7 @@ entities:
           max: 4500
   - entity: sensor
     name: Runtime
-    icon: "mdi:chart-timeline-variant-shimmer"
+    class: duration
     hidden: true
     dps:
       - id: 117