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

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 <engzerjun@gmail.com>

server/etcdserver/api/rafthttp/pipeline.go

@@ -18,7 +18,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
-	"io/ioutil"
+	"io"
 	"runtime"
 	"sync"
 	"time"
@@ -154,7 +154,7 @@ func (p *pipeline) post(data []byte) (err error) {
 		return err
 	}
 	defer resp.Body.Close()
-	b, err := ioutil.ReadAll(resp.Body)
+	b, err := io.ReadAll(resp.Body)
 	if err != nil {
 		p.picker.unreachable(u)
 		return err