*: 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/backend_test.go

@@ -16,7 +16,7 @@ package backend_test
 
 import (
 	"fmt"
-	"io/ioutil"
+	"os"
 	"reflect"
 	"testing"
 	"time"
@@ -59,7 +59,7 @@ func TestBackendSnapshot(t *testing.T) {
 	b.ForceCommit()
 
 	// write snapshot to a new file
-	f, err := ioutil.TempFile(t.TempDir(), "etcd_backend_test")
+	f, err := os.CreateTemp(t.TempDir(), "etcd_backend_test")
 	if err != nil {
 		t.Fatal(err)
 	}