refactor: move from io/ioutil to io and os package

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>

cio/io_unix.go

@@ -22,7 +22,6 @@ package cio
 import (
 	"context"
 	"io"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sync"
@@ -39,7 +38,7 @@ func NewFIFOSetInDir(root, id string, terminal bool) (*FIFOSet, error) {
 			return nil, err
 		}
 	}
-	dir, err := ioutil.TempDir(root, "")
+	dir, err := os.MkdirTemp(root, "")
 	if err != nil {
 		return nil, err
 	}