all: remove leftover use of deprecated ioutil

[AlexanderYastrebov]

Background

ioutil is deprecated, see https://go.dev/doc/go1.16#ioutil

Follow up on #634

Checklist

• [x] Git commit messages conform to community standards.
• [x] Each Git commit represents meaningful milestones or atomic units of work.
• [x] Changed or added code is covered by appropriate tests.

[tsenart]

Thanks! Merged in 712ff2e