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

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.

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

net/dns/resolver/forwarder.go

@@ -11,7 +11,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"math/rand"
 	"net"
 	"net/http"
@@ -474,7 +473,7 @@ func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client,
 		metricDNSFwdDoHErrorCT.Add(1)
 		return nil, fmt.Errorf("unexpected response Content-Type %q", ct)
 	}
-	res, err := ioutil.ReadAll(hres.Body)
+	res, err := io.ReadAll(hres.Body)
 	if err != nil {
 		metricDNSFwdDoHErrorBody.Add(1)
 	}