Replace most usages of ioutil with the underlying function

The ioutil functions are deprecated since Go 1.17 and only wrap another
library function. Thus directly call the underlying function.

This commit only mechanically replaces the function calls.

internal/textfile/read.go

@@ -4,7 +4,7 @@ package textfile
 
 import (
 	"bytes"
-	"io/ioutil"
+	"os"
 
 	"golang.org/x/text/encoding/unicode"
 )
@@ -34,7 +34,7 @@ func Decode(data []byte) ([]byte, error) {
 
 // Read returns the contents of the file, converted to UTF-8, stripped of any BOM.
 func Read(filename string) ([]byte, error) {
-	data, err := ioutil.ReadFile(filename)
+	data, err := os.ReadFile(filename)
 	if err != nil {
 		return nil, err
 	}

internal/textfile/read_test.go

@@ -3,7 +3,6 @@ package textfile
 import (
 	"bytes"
 	"encoding/hex"
-	"io/ioutil"
 	"os"
 	"testing"
 )
@@ -13,7 +12,7 @@ import (
 func writeTempfile(t testing.TB, data []byte) (string, func()) {
 	t.Helper()
 
-	f, err := ioutil.TempFile("", "restic-test-textfile-read-")
+	f, err := os.CreateTemp("", "restic-test-textfile-read-")
 	if err != nil {
 		t.Fatal(err)
 	}