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>

cmd/hello/hello.go

@@ -13,7 +13,6 @@ import (
 	"errors"
 	"flag"
 	"html/template"
-	"io/ioutil"
 	"log"
 	"net/http"
 	"os"
@@ -106,7 +105,7 @@ func devMode() bool { return *httpsAddr == "" && *httpAddr != "" }
 
 func getTmpl() (*template.Template, error) {
 	if devMode() {
-		tmplData, err := ioutil.ReadFile("hello.tmpl.html")
+		tmplData, err := os.ReadFile("hello.tmpl.html")
 		if os.IsNotExist(err) {
 			log.Printf("using baked-in template in dev mode; can't find hello.tmpl.html in current directory")
 			return tmpl, nil