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/armor/armor_test.go

@@ -7,7 +7,7 @@ package armor
 import (
 	"bytes"
 	"hash/adler32"
-	"io/ioutil"
+	"io"
 	"testing"
 )
 
@@ -29,7 +29,7 @@ func TestDecodeEncode(t *testing.T) {
 		t.Errorf("result.Header: got:%#v", result.Header)
 	}
 
-	contents, err := ioutil.ReadAll(result.Body)
+	contents, err := io.ReadAll(result.Body)
 	if err != nil {
 		t.Error(err)
 	}