Fix default value for discharge current limit

If the default type is an int, setting to a fractional value is not possible.

Presumably this is because in
`target.DischargeCurrentLimit = source["discharge_current_limit"] | BATTERY_DISCHARGE_CURRENT_LIMIT;`
the JSON library uses the default to force the expected type and on type mismatch the default is used.

As per https://arduinojson.org/v7/api/jsonvariant/or/:
`defaultValue: the value to return if the JsonVariant is null or incompatible with the requested type.`

include/defaults.h

@@ -155,7 +155,7 @@
 #define BATTERY_JKBMS_INTERFACE 0
 #define BATTERY_JKBMS_POLLING_INTERVAL 5
 #define BATTERY_ENABLE_DISCHARGE_CURRENT_LIMIT false
-#define BATTERY_DISCHARGE_CURRENT_LIMIT 0
+#define BATTERY_DISCHARGE_CURRENT_LIMIT 0.0
 #define BATTERY_USE_BATTERY_REPORTED_DISCHARGE_CURRENT_LIMIT false
 
 #define HUAWEI_ENABLED false