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

The io/ioutil package has been deprecated in Go 1.16. 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>

client/image_history_test.go

@@ -5,7 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"strings"
 	"testing"
@@ -47,7 +47,7 @@ func TestImageHistory(t *testing.T) {
 
 			return &http.Response{
 				StatusCode: http.StatusOK,
-				Body:       ioutil.NopCloser(bytes.NewReader(b)),
+				Body:       io.NopCloser(bytes.NewReader(b)),
 			}, nil
 		}),
 	}