all: clean up obsolete +build lines and io/ioutil uses [generated]

The module go directive is at 1.20, so modernize it a bit by
applying changes generated by the script below with go1.21.1.

[git-generate]
go fix ./...
gofmt -r 'ioutil.Discard -> io.Discard' -w .
gofmt -r 'ioutil.NopCloser -> io.NopCloser' -w .
gofmt -r 'ioutil.ReadAll -> io.ReadAll' -w .
gofmt -r '"ioutil.ReadAll: %v" -> "io.ReadAll: %v"' -w .
gofmt -r 'ioutil.ReadDir -> os.ReadDir' -w .
gofmt -r 'ioutil.ReadFile -> os.ReadFile' -w .
gofmt -r 'ioutil.TempDir -> os.MkdirTemp' -w .
gofmt -r 'ioutil.TempFile -> os.CreateTemp' -w .
gofmt -r 'ioutil.WriteFile -> os.WriteFile' -w .
goimports -w .

Change-Id: I145b27756e7c6101e992747ee822e7237df4cb91
Reviewed-on: https://go-review.googlesource.com/c/build/+/527016
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: dmitshur

autocertcache/autocertcache.go

@@ -8,7 +8,7 @@ package autocertcache
 
 import (
 	"context"
-	"io/ioutil"
+	"io"
 
 	"cloud.google.com/go/storage"
 	"golang.org/x/crypto/acme/autocert"
@@ -40,7 +40,7 @@ func (c *gcsAutocertCache) Get(ctx context.Context, key string) ([]byte, error)
 		return nil, err
 	}
 	defer rd.Close()
-	return ioutil.ReadAll(rd)
+	return io.ReadAll(rd)
 }
 
 func (c *gcsAutocertCache) Put(ctx context.Context, key string, data []byte) error {

buildlet/buildlet.go

@@ -8,7 +8,7 @@ import (
 	"context"
 	"crypto/tls"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"net/http"
 	"time"
@@ -139,7 +139,7 @@ func probeBuildlet(ctx context.Context, buildletURL string, opts *VMOpts) error
 	if err != nil {
 		return fmt.Errorf("error probe buildlet %s: %w", buildletURL, err)
 	}
-	ioutil.ReadAll(res.Body)
+	io.ReadAll(res.Body)
 	res.Body.Close()
 	if res.StatusCode != http.StatusOK {
 		return fmt.Errorf("buildlet returned HTTP status code %d for %s", res.StatusCode, buildletURL)