refactor: 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>

cmd/wails/15_migrate.go

@@ -3,7 +3,6 @@ package main
 import (
 	"bufio"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
@@ -305,7 +304,7 @@ func updateWailsVersion(currentVersion, latestVersion *semver.Version) error {
 	new := fmt.Sprintf("%s v%s", wailsModule, latestVersion)
 
 	goMod = strings.Replace(goMod, old, new, -1)
-	err := ioutil.WriteFile(goModFile, []byte(goMod), 0600)
+	err := os.WriteFile(goModFile, []byte(goMod), 0600)
 	if err != nil {
 		checkSpinner.Error()
 		return err
@@ -343,7 +342,7 @@ func patchMainJS() error {
 	newStartLine := `Wails.Init`
 	mainJSContents = strings.Replace(mainJSContents, oldStartLine, newStartLine, -1)
 
-	err := ioutil.WriteFile(mainJSFile, []byte(mainJSContents), 0600)
+	err := os.WriteFile(mainJSFile, []byte(mainJSContents), 0600)
 	if err != nil {
 		checkSpinner.Error()
 		return err

cmd/wails/9_issue.go

@@ -2,7 +2,7 @@ package main
 
 import (
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"net/url"
 	"os"
@@ -112,7 +112,7 @@ To help you in this process, we will ask for some information, add Go/Wails deta
 			os.Exit(1)
 		}
 		defer resp.Body.Close()
-		template, _ := ioutil.ReadAll(resp.Body)
+		template, _ := io.ReadAll(resp.Body)
 		body := string(template)
 		body = "**Description**\n" + (strings.Split(body, "**Description**")[1])
 		fullURL := "https://github.com/wailsapp/wails/issues/new?"