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/restorer/filerestorer_test.go

@@ -5,7 +5,7 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"io/ioutil"
+	"os"
 	"testing"
 
 	"github.com/restic/restic/internal/crypto"
@@ -171,7 +171,7 @@ func restoreAndVerify(t *testing.T, tempdir string, content []TestFile, files ma
 func verifyRestore(t *testing.T, r *fileRestorer, repo *TestRepo) {
 	for _, file := range r.files {
 		target := r.targetPath(file.location)
-		data, err := ioutil.ReadFile(target)
+		data, err := os.ReadFile(target)
 		if err != nil {
 			t.Errorf("unable to read file %v: %v", file.location, err)
 			continue