Korex kettle: disable keep warm temp when irrelevant

Avoid some confusion about which temperature to set by only enabling
the "Keep warm temperature" when the target temperature is set to boil,
and keep warm is enabled.

PR #3301

custom_components/tuya_local/devices/korex_axwf306n_smart_kettle.yaml

@@ -194,3 +194,13 @@ entities:
             value: "55 C / 131 F"
           - dps_val: "10"
             value: "50 C / 122 F"
+      - id: 101
+        type: string
+        name: available
+        mapping:
+          - dps_val: "0"
+            value_redirect: keep_warm
+          - value: false
+      - id: 103
+        type: boolean
+        name: keep_warm