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/fetcher.go

@@ -21,7 +21,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 	"strings"
@@ -197,7 +196,7 @@ func (r dockerFetcher) open(ctx context.Context, req *request, mediatype string,
 
 			// Discard up to offset
 			// Could use buffer pool here but this case should be rare
-			n, err := io.Copy(ioutil.Discard, io.LimitReader(resp.Body, offset))
+			n, err := io.Copy(io.Discard, io.LimitReader(resp.Body, offset))
 			if err != nil {
 				return nil, errors.Wrap(err, "failed to discard to offset")
 			}