Run modernize to rewrite some older go constructs to newer ones

Mostly using slice.Sort, using min/max, slices.Concat, range of int and
fmt.Appendf for byte slices instead of strings.

dsn/dsn.go

@@ -340,10 +340,7 @@ func (m *Message) Compose(log mlog.Log, smtputf8 bool) ([]byte, error) {
 			data := base64.StdEncoding.EncodeToString(headers)
 			for len(data) > 0 {
 				line := data
-				n := len(line)
-				if n > 78 {
-					n = 78
-				}
+				n := min(len(line), 78)
 				line, data = data[:n], data[n:]
 				if _, err := origp.Write([]byte(line + "\r\n")); err != nil {
 					return nil, err

dsn/parse.go

@@ -14,6 +14,7 @@ import (
 	"github.com/mjl-/mox/message"
 	"github.com/mjl-/mox/mlog"
 	"github.com/mjl-/mox/smtp"
+	"slices"
 )
 
 // Parse reads a DSN message.
@@ -217,15 +218,9 @@ func parseRecipientHeader(mr *textproto.Reader, utf8 bool) (Recipient, error) {
 		case "Action":
 			a := Action(strings.ToLower(v))
 			actions := []Action{Failed, Delayed, Delivered, Relayed, Expanded}
-			var ok bool
-			for _, x := range actions {
-				if a == x {
-					ok = true
-					r.Action = a
-					break
-				}
-			}
-			if !ok {
+			if slices.Contains(actions, a) {
+				r.Action = a
+			} else {
 				err = fmt.Errorf("unrecognized action %q", v)
 			}
 		case "Status":