CA returns number overflow on ipv6 stats

The issue was found during 1.13 sanity checks by @slawek.

I have observed a weird error in logs:

stork-1130-agent-kea6-1              | INFO  COMMAND_RECEIVED Received command 'statistic-get-all'
stork-1130-agent-kea6-1              | INFO  CTRL_AGENT_COMMAND_RECEIVED command statistic-get-all received from remote address 127.0.0.1
stork-1130-agent-kea6-1              | INFO  COMMAND_RECEIVED Received command 'statistic-get-all'
stork-1130-agent-kea6-1              | time="2023-10-10 12:40:51" level="error" msg="Failed to parse responses from Kea: response result from Kea != 0: 1, text: internal server error: unable to parse server's answer to the forwarded message: Number overflow: 36893488147419103232 in <wire>:0:8422" file="  promkeaexporter.go:850  "
stork-1130-agent-kea6-1              | time="2023-10-10 12:40:51" level="error" msg="Some errors were encountered while collecting stats from Kea: response result from Kea != 0: 1, text: internal server error: unable to parse server's answer to the forwarded message: Number overflow: 36893488147419103232 in <wire>:0:8422\nisc.org/stork/agent.(*GetAllStatisticsResponse).UnmarshalJSON\n\tisc.org/stork/agent/promkeaexporter.go:149\nencoding/json.(*decodeState).array\n\tencoding/json/decode.go:507\nencoding/json.(*decodeState).value\n\tencoding/json/decode.go:364\nencoding/json.(*decodeState).unmarshal\n\tencoding/json/decode.go:181\nencoding/json.Unmarshal\n\tencoding/json/decode.go:108\nisc.org/stork/agent.(*PromKeaExporter).collectStats\n\tisc.org/stork/agent/promkeaexporter.go:847\nisc.org/stork/agent.(*PromKeaExporter).statsCollectorLoop\n\tisc.org/stork/agent/promkeaexporter.go:710\nruntime.goexit\n\truntime/asm_amd64.s:1650" file="  promkeaexporter.go:712  "