Upgrade Caddy version to restore xcaddy build

[skehlet]

Hello, running xcaddy build --with github.com/codegangsta/caddy-nats yields a strange error:

panic: internal error: can't find reason for requirement on github.com/google/pprof@v0.0.0-20210407192527-94a9f03dee38

I chased this down, there's more info here and here.

It seems like just upgrading the github.com/caddyserver/caddy/v2 dependency in go.mod to 2.6.3 or the latest 2.6.4 is enough to get past this weird issue. Could you make this change for us please?

Thanks so much. Love your Rethink Connectivity videos.