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