config: Change ARIN healthcheck URL

[awilfox]

The tested POC handle doesn't exist any more; change to ours.

Seems to fix the healthcheck locally:

"whois-arin-api":{"message":"HTTP check successful","success":true,"time":0.2756953889038414}

[anarchivist]

r+; i don't mind this as a fix, but i think the alerts are still being caused by the service being unavailable:

[awilfox]

Ah, interesting. Must have been lucky timing. Still probably better to rely on our own POC handle though.