all: replace io/ioutil with io and os package

For golang/go#45557

Change-Id: I447530cc66896aef7a8d528ccb8d095b80e3cf47
GitHub-Last-Rev: 5f385ff46487ac318bd1147cdbbd26bb0ffd0426
GitHub-Pull-Request: golang/crypto#230
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/430797
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

openpgp/write_test.go

@@ -7,7 +7,6 @@ package openpgp
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
 	"testing"
 	"time"
 
@@ -245,7 +244,7 @@ func TestEncryption(t *testing.T) {
 			}
 		}
 
-		plaintext, err := ioutil.ReadAll(md.UnverifiedBody)
+		plaintext, err := io.ReadAll(md.UnverifiedBody)
 		if err != nil {
 			t.Errorf("#%d: error reading encrypted contents: %s", i, err)
 			continue
@@ -342,7 +341,7 @@ func TestSigning(t *testing.T) {
 			t.Errorf("#%d: failed to find the signing Entity", i)
 		}
 
-		plaintext, err := ioutil.ReadAll(md.UnverifiedBody)
+		plaintext, err := io.ReadAll(md.UnverifiedBody)
 		if err != nil {
 			t.Errorf("#%d: error reading contents: %v", i, err)
 			continue