Remove deprecated ioutil calls

Replaced with `os` package. Also uses t.TempDir in tests over MkDirTemp

out/main.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
@@ -23,7 +22,7 @@ func buildMessage(alert Alert, m concourse.BuildMetadata, path string) *slack.Me
 	// Open and read message file if set
 	if alert.MessageFile != "" {
 		file := filepath.Join(path, alert.MessageFile)
-		f, err := ioutil.ReadFile(file)
+		f, err := os.ReadFile(file)
 
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "error reading message_file: %v\nwill default to message instead\n", err)
@@ -35,7 +34,7 @@ func buildMessage(alert Alert, m concourse.BuildMetadata, path string) *slack.Me
 	// Open and read channel file if set
 	if alert.ChannelFile != "" {
 		file := filepath.Join(path, alert.ChannelFile)
-		f, err := ioutil.ReadFile(file)
+		f, err := os.ReadFile(file)
 
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "error reading channel_file: %v\nwill default to channel instead\n", err)
@@ -47,7 +46,7 @@ func buildMessage(alert Alert, m concourse.BuildMetadata, path string) *slack.Me
 	// Open and read text file if set
 	if alert.TextFile != "" {
 		file := filepath.Join(path, alert.TextFile)
-		f, err := ioutil.ReadFile(file)
+		f, err := os.ReadFile(file)
 
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "error reading text_file: %v\nwill default to text instead\n", err)

out/main_test.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -354,14 +353,9 @@ func TestBuildMessage(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			path := ""
 			if c.alert.MessageFile != "" || c.alert.ChannelFile != "" {
-				dir, err := ioutil.TempDir("", "example")
-				if err != nil {
-					t.Fatal(err)
-				}
-				path = dir
+				path = t.TempDir()
 
-				defer os.RemoveAll(dir)
-				if err := ioutil.WriteFile(filepath.Join(dir, "test_file"), []byte("filecontents"), 0666); err != nil {
+				if err := os.WriteFile(filepath.Join(path, "test_file"), []byte("filecontents"), 0666); err != nil {
 					t.Fatal(err)
 				}
 			}