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/packet/userattribute.go

@@ -9,7 +9,6 @@ import (
 	"image"
 	"image/jpeg"
 	"io"
-	"io/ioutil"
 )
 
 const UserAttrImageSubpacket = 1
@@ -56,7 +55,7 @@ func NewUserAttribute(contents ...*OpaqueSubpacket) *UserAttribute {
 
 func (uat *UserAttribute) parse(r io.Reader) (err error) {
 	// RFC 4880, section 5.13
-	b, err := ioutil.ReadAll(r)
+	b, err := io.ReadAll(r)
 	if err != nil {
 		return
 	}