line up with offical tuya integration to make filter_life a diagnostic property

custom_components/tuya_local/devices/soleusair_air_purifier.yaml

@@ -25,6 +25,7 @@ secondary_entities:
   - entity: sensor
     name: filter_life
     class: duration
+    category: diagnostic
     dps:
       - id: 5
         type: integer