all: Use context in lib/dialer (#6177)

* all: Use context in lib/dialer

* a bit slimmer

* https://github.com/syncthing/syncthing/pull/5753

* bot

* missed adding debug.go

* errors.Cause

* simultaneous dialing

* anti-leak

lib/upgrade/upgrade_supported.go

@@ -66,8 +66,8 @@ const (
 var insecureHTTP = &http.Client{
 	Timeout: readTimeout,
 	Transport: &http.Transport{
-		Dial:  dialer.Dial,
-		Proxy: http.ProxyFromEnvironment,
+		DialContext: dialer.DialContext,
+		Proxy:       http.ProxyFromEnvironment,
 		TLSClientConfig: &tls.Config{
 			InsecureSkipVerify: true,
 		},