refactor: move from io/ioutil to io and os package

The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

remotes/docker/pusher.go

@@ -19,7 +19,6 @@ package docker
 import (
 	"context"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 	"strings"
@@ -263,7 +262,7 @@ func (p dockerPusher) push(ctx context.Context, desc ocispec.Descriptor, ref str
 
 	pr, pw := io.Pipe()
 	respC := make(chan response, 1)
-	body := ioutil.NopCloser(pr)
+	body := io.NopCloser(pr)
 
 	req.body = func() (io.ReadCloser, error) {
 		if body == nil {