Korex kettle: add step for temperature

Ensure the HA UI changes temperature in the correct step size.

Issue raised in comment on #3301

custom_components/tuya_local/devices/korex_axwf306n_smart_kettle.yaml

@@ -31,7 +31,8 @@ entities:
           min: 113
           max: 212
         mapping:
-          - constraint: keep_warm
+          - step: 9
+            constraint: keep_warm
             conditions:
               - dps_val: false
                 mapping: