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>

cmd/gen-manpages/main.go

@@ -19,7 +19,6 @@ package main
 import (
 	"flag"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -62,7 +61,7 @@ func run() error {
 		return err
 	}
 	_ = os.MkdirAll(dir, os.ModePerm)
-	if err := ioutil.WriteFile(filepath.Join(dir, fmt.Sprintf("%s.%s", name, section)), []byte(data), 0644); err != nil {
+	if err := os.WriteFile(filepath.Join(dir, fmt.Sprintf("%s.%s", name, section)), []byte(data), 0644); err != nil {
 		return err
 	}
 	return nil