all: Remove usage of deprecated io/ioutil (#7971)

As of Go 1.16 io/ioutil is deprecated. This replaces usage with the
corresponding functions in package os and package io.

lib/fs/basicfs_watch_test.go

@@ -13,7 +13,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -530,7 +529,7 @@ func renameTestFile(name string, old string, new string) {
 func modifyTestFile(name string, file string, content string) {
 	joined := filepath.Join(testDirAbs, name, file)
 
-	err := ioutil.WriteFile(joined, []byte(content), 0755)
+	err := os.WriteFile(joined, []byte(content), 0755)
 	if err != nil {
 		panic(fmt.Sprintf("Failed to modify test file %s: %s", joined, err))
 	}