Remove deprecated io/ioutil package from the source code

[alexandear]

Is your feature request related to a problem? Please describe.

Package io/ioutil deprecated since Go 1.16:

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.

Describe the solution you'd like

• Replace ioutil.Discard with io.Discard.
• Replace ioutil.ReadFile, ioutil.WriteFile, ioutil.ReadDir with os.ReadFile, os.WriteFile, os.ReadDir.

[mgechev]

Opened #825 and will merge once @chavacava approves.