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>

diff/windows/windows.go

@@ -24,7 +24,6 @@ import (
 	"encoding/base64"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"time"
 
 	winio "github.com/Microsoft/go-winio"
@@ -150,7 +149,7 @@ func (s windowsDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mounts
 	}
 
 	// Read any trailing data
-	if _, err := io.Copy(ioutil.Discard, rc); err != nil {
+	if _, err := io.Copy(io.Discard, rc); err != nil {
 		return emptyDesc, err
 	}