refactor: move from io/ioutil to io and os packages (#5266)

The io/ioutil package has been deprecated as of Go 1.16 [1]. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.

[1]: https://golang.org/doc/go1.16#ioutil
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

pkg/simple/client/monitoring/metricsserver/metricsserver_test.go

@@ -2,11 +2,10 @@ package metricsserver
 
 import (
 	"fmt"
+	"os"
 	"testing"
 	"time"
 
-	"io/ioutil"
-
 	"github.com/google/go-cmp/cmp"
 	jsoniter "github.com/json-iterator/go"
 
@@ -514,7 +513,7 @@ func TestGetNamedMetricsOverTime(t *testing.T) {
 }
 
 func jsonFromFile(expectedFile string, expectedJsonPtr interface{}) error {
-	json, err := ioutil.ReadFile(fmt.Sprintf("./testdata/%s", expectedFile))
+	json, err := os.ReadFile(fmt.Sprintf("./testdata/%s", expectedFile))
 	if err != nil {
 		return err
 	}