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,
 }

pkg/cmd/grafana-cli/services/services.go

@@ -30,6 +30,7 @@ func Init(version string) {
 		DialContext: (&net.Dialer{
 			Timeout:   30 * time.Second,
 			KeepAlive: 30 * time.Second,
+			DualStack: true,
 		}).DialContext,
 		MaxIdleConns:          100,
 		IdleConnTimeout:       90 * time.Second,

pkg/components/imguploader/webdavuploader.go

@@ -23,7 +23,8 @@ type WebdavUploader struct {
 var netTransport = &http.Transport{
 	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
-		Timeout: 60 * time.Second,
+		Timeout:   60 * time.Second,
+		DualStack: true,
 	}).Dial,
 	TLSHandshakeTimeout: 5 * time.Second,
 }

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,

pkg/services/notifications/webhook.go

@@ -27,7 +27,8 @@ type Webhook struct {
 var netTransport = &http.Transport{
 	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
-		Timeout: 30 * time.Second,
+		Timeout:   30 * time.Second,
+		DualStack: true,
 	}).Dial,
 	TLSHandshakeTimeout: 5 * time.Second,
 }