Replace uses of ioutil

ioutil package was deprecated in Go 1.16, replace all uses with their
respective replacements.

This patch was generated with a combination of `gofmt -r`, `eg`, and
manually (for `ioutil.ReadDir`).

internal/courier/fakeserver_test.go

@@ -4,7 +4,6 @@ import (
 	"bufio"
 	"crypto/tls"
 	"crypto/x509"
-	"io/ioutil"
 	"net"
 	"net/textproto"
 	"os"
@@ -62,7 +61,7 @@ func (s *FakeServer) rootCA() *x509.CertPool {
 	s.t.Helper()
 	pool := x509.NewCertPool()
 	path := s.tmpDir + "/cert.pem"
-	data, err := ioutil.ReadFile(path)
+	data, err := os.ReadFile(path)
 	if err != nil {
 		s.t.Fatalf("error reading cert %q: %v", path, err)
 	}

internal/courier/mda_test.go

@@ -2,7 +2,6 @@ package courier
 
 import (
 	"bytes"
-	"io/ioutil"
 	"os"
 	"testing"
 	"time"
@@ -25,7 +24,7 @@ func TestMDA(t *testing.T) {
 		t.Fatalf("Deliver: %v", err)
 	}
 
-	data, err := ioutil.ReadFile(dir + "/to")
+	data, err := os.ReadFile(dir + "/to")
 	if err != nil || !bytes.Equal(data, []byte("data")) {
 		t.Errorf("Invalid data: %q - %v", string(data), err)
 	}