Replace deprecated ioutil pkg with os & io

[abhinavnair]

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.

So replacing all usage of ioutil pkg with io & os.

[apparentlymart]

Hi @abhinavnair! Thanks for working on this.

For this kind of situation where the recommendations have changed upstream since we originally wrote some code, we prefer to handle the updates gradually whenever we'd be making other changes to each subsystem, rather than making broad, cross-cutting changes like this. A major reason for that is that we often need to backport changes between different feature branches, and that gets more difficult if there are lots of small non-functionality-related differences like these between the content of the different branches.

Because of that, I don't expect that we would merge a change like this focused only on replacing calls to some functions with calls to equivalent functions. Although as you said the old functions are not recommended for new code, we do intend to keep using them in existing code until we are making other changes to these codepaths that have sufficient benefit to justify the cost of it being harder to backport bug fixes into earlier release branches.

While I do appreciate your efforts working on this, I see that you're facing some test failures and I wanted to let you know our position on this before you spend time trying to resolve those, since I don't expect we would be able to merge this even if you do get the tests to pass.

Thanks again!

[abhinavnair]

Hi @apparentlymart . Thanks for informing me. I understand the reason for the choice and fully support it. I shall close this PR for now. Hope to contribute in a more meaningful way in the future 🙂

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