java-generator type=string format=date-time serializes with format that is not compatible with k8s

Serialization code created by java-generator for type=string with format=date-time uses a timezone which is not compatible with kubernetes.

Example error message:
```
Invalid value: "2025-05-30T23:04:44Zulu": startTime in body must be of type date-time
```

See also:
https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html