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/apiserver/authentication/token/issuer.go

@@ -25,7 +25,7 @@ import (
 	"errors"
 	"fmt"
 	"hash/fnv"
-	"io/ioutil"
+	"os"
 	"time"
 
 	"gopkg.in/square/go-jose.v2"
@@ -264,7 +264,7 @@ func loadSignKey(options *authentication.Options) (*rsa.PrivateKey, string, erro
 	var err error
 
 	if options.OAuthOptions.SignKey != "" {
-		signKeyData, err = ioutil.ReadFile(options.OAuthOptions.SignKey)
+		signKeyData, err = os.ReadFile(options.OAuthOptions.SignKey)
 		if err != nil {
 			klog.Errorf("issuer: failed to read private key file %s: %v", options.OAuthOptions.SignKey, err)
 			return nil, "", err