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/identityprovider/github/github.go

@@ -20,7 +20,7 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/json"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"time"
 
@@ -190,7 +190,7 @@ func (g *github) IdentityExchangeCallback(req *http.Request) (identityprovider.I
 		return nil, err
 	}
 
-	data, err := ioutil.ReadAll(resp.Body)
+	data, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err
 	}