chore: update to go1.21rc2, drop support for go1.19

component/dialer/error.go

@@ -2,17 +2,9 @@ package dialer
 
 import (
 	"errors"
-
-	E "github.com/sagernet/sing/common/exceptions"
 )
 
 var (
 	ErrorNoIpAddress           = errors.New("no ip address")
 	ErrorInvalidedNetworkStack = errors.New("invalided network stack")
 )
-
-func errorsJoin(errs ...error) error {
-	// compatibility with golang<1.20
-	// maybe use errors.Join(errs...) is better after we drop the old version's support
-	return E.Errors(errs...)
-}