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/strelaypoolsrv/main.go

@@ -11,7 +11,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
@@ -560,7 +559,7 @@ func limit(addr string, cache *lru.Cache, lock sync.Mutex, intv time.Duration, b
 }
 
 func loadRelays(file string) []*relay {
-	content, err := ioutil.ReadFile(file)
+	content, err := os.ReadFile(file)
 	if err != nil {
 		log.Println("Failed to load relays: " + err.Error())
 		return nil
@@ -598,11 +597,11 @@ func saveRelays(file string, relays []*relay) error {
 	for _, relay := range relays {
 		content += relay.uri.String() + "\n"
 	}
-	return ioutil.WriteFile(file, []byte(content), 0777)
+	return os.WriteFile(file, []byte(content), 0777)
 }
 
 func createTestCertificate() tls.Certificate {
-	tmpDir, err := ioutil.TempDir("", "relaypoolsrv")
+	tmpDir, err := os.MkdirTemp("", "relaypoolsrv")
 	if err != nil {
 		log.Fatal(err)
 	}