*: 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>

client/pkg/fileutil/lock_test.go

@@ -15,14 +15,13 @@
 package fileutil
 
 import (
-	"io/ioutil"
 	"os"
 	"testing"
 	"time"
 )
 
 func TestLockAndUnlock(t *testing.T) {
-	f, err := ioutil.TempFile("", "lock")
+	f, err := os.CreateTemp("", "lock")
 	if err != nil {
 		t.Fatal(err)
 	}