Replace deprecated ioutil with io and os

[alexandear]

This PR removes usages of io/ioutil.

The package io/ioutil is 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.

Changes

• Replace ioutil.ReadAll, ioutil.Discard, ioutil.NopCloser with io.ReadAll, io.Discard, io.NopCloser.
• Replace ioutil.ReadFile, ioutil.WriteFile, ioutil.ReadDir with os.ReadFile, os.WriteFile, os.ReadDir.
• Replace ioutil.TempFile with os.CreateTemp.
• Replace ioutil.TempDir with os.MkdirTemp.

Tests only

• Replace ioutil.TempDir with t.TempDir(). T.TempDir automatically clears directory.
• Refactor initFsTests by replacing ioutil.TempDir with t.TempDir and using t.Cleanup instead of manual cleanup.
• Remove unused createEmptyTempDir and deleteTempDir after refactoring to t.TempDir.

[bep]

Thanks for this. This needs to wait before I can merge; I have some bigger change sets in the merge pipeline in the near future, and I fear that these changes may create lots of potential merge conflicts.

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All committers have signed the CLA.

[alexandear]

Created #10768 about data race in tests.

[bep]

@alexandear I'm going to merge this once the tests go green, but a general remark for the future: If you have a PR that states that it does one isolated thing (ioutil => os) it would be good if it also doesn't do some refactoring of something totally unrelated. I know it's tempting and I'm certainly throwing stones in a glass house here, but it makes PRs like these much harder to reason about than they should.

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