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/apply/apply.go

@@ -19,7 +19,6 @@ package apply
 import (
 	"context"
 	"io"
-	"io/ioutil"
 	"time"
 
 	"github.com/containerd/containerd/content"
@@ -99,7 +98,7 @@ func (s *fsApplier) 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
 	}