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/lcow/lcow.go

@@ -21,7 +21,6 @@ package lcow
 import (
 	"context"
 	"io"
-	"io/ioutil"
 	"os"
 	"path"
 	"runtime"
@@ -166,7 +165,7 @@ func (s windowsLcowDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mou
 	outFile.Close()
 
 	// 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
 	}