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.

lib/discover/global.go

@@ -14,7 +14,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -173,7 +172,7 @@ func (c *globalClient) Lookup(ctx context.Context, device protocol.DeviceID) (ad
 		return nil, err
 	}
 
-	bs, err := ioutil.ReadAll(resp.Body)
+	bs, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err
 	}

lib/discover/global_test.go

@@ -9,7 +9,7 @@ package discover
 import (
 	"context"
 	"crypto/tls"
-	"io/ioutil"
+	"io"
 	"net"
 	"net/http"
 	"strings"
@@ -232,7 +232,7 @@ func (s *fakeDiscoveryServer) handler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if r.Method == "POST" {
-		s.announce, _ = ioutil.ReadAll(r.Body)
+		s.announce, _ = io.ReadAll(r.Body)
 		w.WriteHeader(204)
 	} else {
 		w.Header().Set("Content-Type", "application/json")