Salt pillar: update Prometheus IP to IPv6

I've switched the Prometheus server to communicate over IPv6 now, so
this needs to be updated to make the nginx configuration correct.

salt/pillar/prod.sls

@@ -5,7 +5,7 @@ ssl_private_key_path: /etc/letsencrypt/live/tildes.net/privkey.pem
 hsts_max_age: 63072000
 nginx_worker_processes: auto
 postgresql_version: 12
-prometheus_ips: ['144.217.93.123']
+prometheus_ips: ['2607:5300:201:3100::6e77']
 site_hostname: tildes.net
 ipv6_address: '2607:5300:0203:2dd8::'
 ipv6_gateway: '2607:5300:0203:2dff:ff:ff:ff:ff'