Set default value of dns_order in Kong gateway to `LAST,A,CNAME,SRV`

[randmonkey]

During troubleshooting of Performance problems in customer's Konnect PoC, we found a DNS problem that it can take a long time (~2s) on DNS resolving. When we change the dns_order and remove its SRV in Kong.conf, the performance became much better.

In Kubernetes, SRV records are used for domain names with named ports, in format _port-name._port-protocol.my-svc.my-namespace.svc.cluster-domain.example, like _http._tcp.kong-proxy.kong.svc.cluster.local. For usual service names in format name.namespace, an A record is returned. When we set hostnames of upstream services, we usually use the name.namespace format. So we should change the default settings of dns_order from LAST,SRV,A,CNAME to LAST,A,SRV,CNAME to give a higher priority on A records.