*: 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/storage/backend/testing/betesting.go

@@ -15,7 +15,7 @@
 package betesting
 
 import (
-	"io/ioutil"
+	"os"
 	"path/filepath"
 	"testing"
 	"time"
@@ -26,7 +26,7 @@ import (
 )
 
 func NewTmpBackendFromCfg(t testing.TB, bcfg backend.BackendConfig) (backend.Backend, string) {
-	dir, err := ioutil.TempDir(t.TempDir(), "etcd_backend_test")
+	dir, err := os.MkdirTemp(t.TempDir(), "etcd_backend_test")
 	if err != nil {
 		panic(err)
 	}