replace depricated io/ioutil

[blue-troy]

Proposed commit message

As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred. Successor functionality in io or os is more performant than the deprecated functionality in io/ioutil.

Checklist

• [x] My code follows the style guidelines of this project
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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