http: remove dualstack since its deprecated (#16940)

Deprecated: Fast Fallback is enabled by default. To
disable, set FallbackDelay to a negative value.

https://golang.org/src/net/dial.go

pkg/services/notifications/webhook.go

@@ -32,8 +32,7 @@ var netTransport = &http.Transport{
 	},
 	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
-		Timeout:   30 * time.Second,
-		DualStack: true,
+		Timeout: 30 * time.Second,
 	}).Dial,
 	TLSHandshakeTimeout: 5 * time.Second,
 }

pkg/services/rendering/http_mode.go

@@ -15,8 +15,7 @@ import (
 var netTransport = &http.Transport{
 	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
-		Timeout:   30 * time.Second,
-		DualStack: true,
+		Timeout: 30 * time.Second,
 	}).Dial,
 	TLSHandshakeTimeout: 5 * time.Second,
 }