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/checker/checker.go

@@ -6,7 +6,6 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"runtime"
 	"sort"
 	"sync"
@@ -556,7 +555,7 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID, blobs []r
 			}
 
 			// read remainder, which should be the pack header
-			hdrBuf, err = ioutil.ReadAll(bufRd)
+			hdrBuf, err = io.ReadAll(bufRd)
 			if err != nil {
 				return err
 			}

internal/checker/checker_test.go

@@ -3,7 +3,6 @@ package checker_test
 import (
 	"context"
 	"io"
-	"io/ioutil"
 	"math/rand"
 	"os"
 	"path/filepath"
@@ -208,7 +207,7 @@ func TestModifiedIndex(t *testing.T) {
 		Name: "90f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
 	}
 
-	tmpfile, err := ioutil.TempFile("", "restic-test-mod-index-")
+	tmpfile, err := os.CreateTemp("", "restic-test-mod-index-")
 	if err != nil {
 		t.Fatal(err)
 	}