Format endpoints based off of IP family when dynamically updating upstream servers through the N+ API

[kate-osborn]

When we added support for ipv6 endpoints, we forgot to update this function. It’s used when we update the upstream servers through the N+ API.

We need to format the endpoints differently when they are ipv6. Like we do here: https://github.com/nginxinc/nginx-gateway-fabric/blob/9840a51202a67fb2d4d6203863a203521d356458/internal/mode/static/nginx/config/upstreams.go#L70