Enable dualstack in every net.Dialer, fixes #9364

Default transport enables it:

* https://github.com/golang/go/blob/d2826d3e06/src/net/http/transport.go#L42-L46

```
    DialContext: (&net.Dialer{
	Timeout:   30 * time.Second,
	KeepAlive: 30 * time.Second,
	DualStack: true,
    }).DialContext,
```

See also: https://github.com/golang/go/issues/15324

pkg/api/app_routes.go

@@ -22,6 +22,7 @@ var pluginProxyTransport = &http.Transport{
 	Dial: (&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
+		DualStack: true,
 	}).Dial,
 	TLSHandshakeTimeout: 10 * time.Second,
 }

pkg/api/grafana_com_proxy.go

@@ -19,6 +19,7 @@ var grafanaComProxyTransport = &http.Transport{
 	Dial: (&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
+		DualStack: true,
 	}).Dial,
 	TLSHandshakeTimeout: 10 * time.Second,
 }