Add ipVersion to IPNetwork

core/src/main/java/be/dnsbelgium/rdap/core/IPNetwork.java
is missing ipVersion field
https://datatracker.ietf.org/doc/html/rfc9083#name-the-ip-network-object-class

```
ipVersion -- a string signifying the IP protocol version of the network: "v4" signifies an IPv4 network, and "v6" signifies an IPv6 network
```