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/models/datasource_cache.go

@@ -53,6 +53,7 @@ func (ds *DataSource) GetHttpTransport() (*http.Transport, error) {
 		Dial: (&net.Dialer{
 			Timeout:   30 * time.Second,
 			KeepAlive: 30 * time.Second,
+			DualStack: true,
 		}).Dial,
 		TLSHandshakeTimeout:   10 * time.Second,
 		ExpectContinueTimeout: 1 * time.Second,