Update usage of deprecated io/ioutil library

[amidam]

What would you like?

I noticed that the project is still using the deprecated io/ioutil library in some parts of the codebase. Since io/ioutil has been deprecated in Go 1.16, it's recommended to update the code to use the newer io package instead.

I propose updating the affected parts of the codebase to use the io package for improved compatibility with future versions of Go.

[peterbourgon]

Go for it.