all: Remove usage of deprecated io/ioutil (#7971)

As of Go 1.16 io/ioutil is deprecated. This replaces usage with the
corresponding functions in package os and package io.

cmd/stsigtool/main.go

@@ -9,7 +9,6 @@ package main
 import (
 	"flag"
 	"io"
-	"io/ioutil"
 	"log"
 	"os"
 
@@ -69,7 +68,7 @@ func gen() {
 }
 
 func sign(keyname, dataname string) {
-	privkey, err := ioutil.ReadFile(keyname)
+	privkey, err := os.ReadFile(keyname)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -95,7 +94,7 @@ func sign(keyname, dataname string) {
 }
 
 func verifyWithFile(signame, dataname, keyname string) {
-	pubkey, err := ioutil.ReadFile(keyname)
+	pubkey, err := os.ReadFile(keyname)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -103,7 +102,7 @@ func verifyWithFile(signame, dataname, keyname string) {
 }
 
 func verifyWithKey(signame, dataname string, pubkey []byte) {
-	sig, err := ioutil.ReadFile(signame)
+	sig, err := os.ReadFile(signame)
 	if err != nil {
 		log.Fatal(err)
 	}