upgrade go to 1.16

Fixes #1510

.travis.yml

 dist: focal
 
 go:
-    - "1.15.x"
+    - "1.16.x"
 
 branches:
   only:

Dockerfile

 ## build Oragono
-FROM golang:1.15-alpine AS build-env
+FROM golang:1.16-alpine AS build-env
 
 RUN apk add --no-cache git make curl sed
 

go.mod

 module github.com/oragono/oragono
 
-go 1.15
+go 1.16
 
 require (
 	code.cloudfoundry.org/bytefmt v0.0.0-20200131002437-cf55d5288a48

irc/config.go

 	"crypto/tls"
 	"errors"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"net"
 	"os"
 
 // LoadRawConfig loads the config without doing any consistency checks or postprocessing
 func LoadRawConfig(filename string) (config *Config, err error) {
-	data, err := ioutil.ReadFile(filename)
+	data, err := os.ReadFile(filename)
 	if err != nil {
 		return nil, err
 	}
 			return err
 		}
 		defer file.Close()
-		contents, err := ioutil.ReadAll(file)
+		contents, err := io.ReadAll(file)
 		if err != nil {
 			return err
 		}

irc/email/dkim.go

 import (
 	"errors"
 	dkim "github.com/toorop/go-dkim"
-	"io/ioutil"
+	"os"
 )
 
 var (
 		if dkim.Selector == "" || dkim.KeyFile == "" {
 			return ErrMissingFields
 		}
-		dkim.keyBytes, err = ioutil.ReadFile(dkim.KeyFile)
+		dkim.keyBytes, err = os.ReadFile(dkim.KeyFile)
 		if err != nil {
 			return err
 		}

irc/import.go

 import (
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
 	"log"
+	"os"
 	"strconv"
 
 	"github.com/tidwall/buntdb"
 }
 
 func ImportDB(config *Config, infile string) (err error) {
-	data, err := ioutil.ReadFile(infile)
+	data, err := os.ReadFile(infile)
 	if err != nil {
 		return
 	}

irc/jwt/extjwt.go

 	"encoding/pem"
 	"errors"
 	"fmt"
-	"io/ioutil"
+	"os"
 	"time"
 
 	"github.com/dgrijalva/jwt-go"
 	t.secretBytes = []byte(t.Secret)
 	t.Secret = ""
 	if t.RSAPrivateKeyFile != "" {
-		keyBytes, err := ioutil.ReadFile(t.RSAPrivateKeyFile)
+		keyBytes, err := os.ReadFile(t.RSAPrivateKeyFile)
 		if err != nil {
 			return err
 		}

irc/languages/languages.go

 import (
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"os"
 	"path/filepath"
 	"sort"
 	"strconv"
 }
 
 func (lm *Manager) loadData(path string) (err error) {
-	files, err := ioutil.ReadDir(path)
+	files, err := os.ReadDir(path)
 	if err != nil {
 		return
 	}
 
 		// load, e.g., `zh-CN.lang.yaml`
 		var data []byte
-		data, err = ioutil.ReadFile(filepath.Join(path, name))
+		data, err = os.ReadFile(filepath.Join(path, name))
 		if err != nil {
 			return
 		}
 		translations := make(map[string]string)
 		for _, translationSuffix := range stringsFileSuffixes {
 			stringsFilePath := filepath.Join(path, prefix+translationSuffix)
-			data, err = ioutil.ReadFile(stringsFilePath)
+			data, err = os.ReadFile(stringsFilePath)
 			if err != nil {
 				continue // skip missing paths
 			}

irc/listeners.go

 			} else {
 				nl.server.logger.Error("internal", "invalid connection type", nl.addr)
 			}
-		} else if err == utils.ErrNetClosing {
+		} else if err == net.ErrClosed {
 			return
 		} else {
 			nl.server.logger.Error("internal", "accept error", nl.addr, err.Error())

irc/utils/proxy.go

 import (
 	"crypto/tls"
 	"encoding/binary"
-	"errors"
 	"io"
 	"net"
 	"strings"
 
 var (
 	ErrBadProxyLine error = &proxyLineError{}
-	// TODO(golang/go#4373): replace this with the stdlib ErrNetClosing
-	ErrNetClosing = errors.New("use of closed network connection")
 )
 
 // ListenerConfig is all the information about how to process
 		if err == nil {
 			conn.Close()
 		}
-		err = ErrNetClosing
+		err = net.ErrClosed
 	}
 	if err != nil {
 		return nil, err