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/cache/file_test.go

@@ -3,7 +3,7 @@ package cache
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"math/rand"
 	"os"
 	"runtime"
@@ -55,7 +55,7 @@ func load(t testing.TB, c *Cache, h restic.Handle) []byte {
 		t.Fatalf("load() returned nil reader")
 	}
 
-	buf, err := ioutil.ReadAll(rd)
+	buf, err := io.ReadAll(rd)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -174,7 +174,7 @@ func TestFileLoad(t *testing.T) {
 				t.Fatal(err)
 			}
 
-			buf, err := ioutil.ReadAll(rd)
+			buf, err := io.ReadAll(rd)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -258,7 +258,7 @@ func TestFileSaveConcurrent(t *testing.T) {
 			}
 			defer func() { _ = f.Close() }()
 
-			read, err := ioutil.ReadAll(f)
+			read, err := io.ReadAll(f)
 			if err == nil && !bytes.Equal(read, data) {
 				err = errors.New("mismatch between Save and Load")
 			}