Replace deprecated package io/ioutil with io and os

[alexandear]

This PR replaces function usages of io/ioutil functions with the same functions from io and os.

Changes

• replace ioutil.ReadFile with os.ReadFile;
• replace ioutil.WriteFile with os.WriteFile;
• replace ioutil.ReadAll with io.ReadAll;
• replace ioutil.Discard with io.Discard;
• replace ioutil.TempFile with os.CreateTemp.

Motivation

io/ioutil is deprecated:

Deprecated: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details.

[andrewsomething]

👍 I've been meaning to clean these up. Thanks for all the help!