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/smtpsrv/conn.go

@@ -8,7 +8,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"math/rand"
 	"net"
 	"net/mail"
@@ -637,7 +636,7 @@ func (c *Conn) DATA(params string) (code int, msg string) {
 	// Create a dot reader, limited to the maximum size.
 	dotr := textproto.NewReader(bufio.NewReader(
 		io.LimitReader(c.reader, c.maxDataSize))).DotReader()
-	c.data, err = ioutil.ReadAll(dotr)
+	c.data, err = io.ReadAll(dotr)
 	if err != nil {
 		if err == io.ErrUnexpectedEOF {
 			// Message is too big already. But we need to keep reading until we see

internal/smtpsrv/fuzz.go

@@ -17,7 +17,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"math/big"
 	"net"
 	"net/textproto"
@@ -235,7 +234,7 @@ func init() {
 	log.Default.Level = log.Error
 
 	// Generate certificates in a temporary directory.
-	tmpDir, err := ioutil.TempDir("", "chasquid_smtpsrv_fuzz:")
+	tmpDir, err := os.MkdirTemp("", "chasquid_smtpsrv_fuzz:")
 	if err != nil {
 		panic(fmt.Errorf("Failed to create temp dir: %v\n", tmpDir))
 	}

internal/smtpsrv/server_test.go

@@ -4,7 +4,6 @@ import (
 	"crypto/tls"
 	"flag"
 	"fmt"
-	"io/ioutil"
 	"net"
 	"net/smtp"
 	"os"
@@ -568,7 +567,7 @@ func realMain(m *testing.M) int {
 		}
 	} else {
 		// Generate certificates in a temporary directory.
-		tmpDir, err := ioutil.TempDir("", "chasquid_test:")
+		tmpDir, err := os.MkdirTemp("", "chasquid_test:")
 		if err != nil {
 			fmt.Printf("Failed to create temp dir: %v\n", tmpDir)
 			return 1