fix(klarta_stor3_purifier): handle 'mild' air quality

Original submitted config did not include mild, but device is reporting it.

I am not sure whether this is an additional level, or it takes the place
of moderate or medium.

For now, link it with moderate (which is actually medium for this device)

Issue #6365

custom_components/tuya_local/devices/klarta_stor3_purifier.yaml

@@ -135,6 +135,8 @@ entities:
             value: excellent
           - dps_val: moderate
             value: good
+          - dps_val: mild
+            value: moderate
           - dps_val: medium
             value: moderate
           - dps_val: unhealthy