Nedis air quality: use new _parts class for VOC sensor.

Since the VOC sensor of this device is using ppm, HA 2024.2 enforces
strongly that you use the volatile_organic_compounds_parts class
rather than the volatile_organic_compounds (which used to list ppm
as an allowed unit, but no longer does).

Issue #1601

custom_components/tuya_local/devices/nedis_airquality.yaml

@@ -38,7 +38,7 @@ secondary_entities:
         mapping:
           - scale: 10
   - entity: sensor
-    class: volatile_organic_compounds
+    class: volatile_organic_compounds_parts
     dps:
       - id: 21
         type: integer