fix(core): serialize duration as strings

Fixes #5615

Since Jackson 2.10, to serialize duration as string we need to switch WRITE_DURATIONS_AS_TIMESTAMPS off as it no longuer use WRITE_DATES_AS_TIMESTAMPS.
I tested and an old flow with a duration as timestamp is still readable so this is a backward compatible change.

core/src/main/java/io/kestra/core/serializers/JacksonMapper.java

@@ -121,6 +121,7 @@ public static ObjectMapper ofIon() {
     private static ObjectMapper configure(ObjectMapper mapper) {
         return mapper
             .configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false)
+            .configure(SerializationFeature.WRITE_DURATIONS_AS_TIMESTAMPS, false)
             .setSerializationInclusion(JsonInclude.Include.NON_NULL)
             .registerModule(new JavaTimeModule())
             .registerModule(new Jdk8Module())