Refactor: move from io/ioutil to io and os package (#2234)

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 <zerjun@beatchain.co>

cmd/core/main_test.go

@@ -17,7 +17,6 @@ limitations under the License.
 package main
 
 import (
-	"io/ioutil"
 	"os"
 	"testing"
 	"time"
@@ -72,7 +71,7 @@ var _ = Describe("test waitSecretVolume", func() {
 			By("add non-empty file")
 			_, err := os.Create(testdir + "/file")
 			Expect(err).NotTo(HaveOccurred())
-			err = ioutil.WriteFile(testdir+"/file", []byte("test"), os.ModeAppend)
+			err = os.WriteFile(testdir+"/file", []byte("test"), os.ModeAppend)
 			Expect(err).NotTo(HaveOccurred())
 			err = waitWebhookSecretVolume(testdir, testTimeout, testInterval)
 			Expect(err).NotTo(HaveOccurred())