Replace deprecated ioutil

As of Go 1.16, the same functionality is now provided by package io or
package os, and those implementations should be preferred in new code.

lib/http/http.go

@@ -7,10 +7,10 @@ import (
 	"crypto/x509"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
+	"os"
 	"strings"
 	"sync"
 	"time"
@@ -187,7 +187,7 @@ func NewServer(listeners, tlsListeners []net.Listener, opt Options) (Server, err
 			return nil, err
 		}
 		certpool := x509.NewCertPool()
-		pem, err := ioutil.ReadFile(opt.ClientCA)
+		pem, err := os.ReadFile(opt.ClientCA)
 		if err != nil {
 			log.Fatalf("Failed to read client certificate authority: %v", err)
 			return nil, err