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

The io/ioutil package has been deprecated as of Go 1.16 [1]. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.

Reference: https://golang.org/doc/go1.16#ioutil
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

ipn/prefs.go

@@ -9,7 +9,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"net/netip"
 	"os"
@@ -618,7 +617,7 @@ func PrefsFromBytes(b []byte) (*Prefs, error) {
 // LoadPrefs loads a legacy relaynode config file into Prefs
 // with sensible migration defaults set.
 func LoadPrefs(filename string) (*Prefs, error) {
-	data, err := ioutil.ReadFile(filename)
+	data, err := os.ReadFile(filename)
 	if err != nil {
 		return nil, fmt.Errorf("LoadPrefs open: %w", err) // err includes path
 	}