refactor: replace deprecated ioutil with io and os

[alexandear]

This PR replaces deprecated in Go 1.16 io/ioutil package with io and os.

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

Replace was done with the help of gofmt and goimports tools:

gofmt -r 'ioutil.WriteFile -> os.WriteFile' -w .
gofmt -r 'ioutil.Discard -> io.Discard' -w .
gofmt -r 'ioutil.ReadAll -> io.ReadAll' -w .
goimports -w .

Changed ioutil.TempDir to t.TempDir in tests.

[CLAassistant]

All committers have signed the CLA.

[alnr]

Hey thanks! Would you mind bumping the runc dependency, too? That oughta make the pipeline green.

[zepatrik]

@alnr see sonatype-nexus-community/nancy#273