Remove deprecated ioutil from generator

[marwan-at-work]

The ioutil std lib package has been deprecated for a few releases now, this PR removes all references to them.

[marioizquierdo]

What is the latest version of Go that would not work with this change? I seems that io.ReadAll was introduced in Go 1.16, meaning that this change would not work with Go 1.15 and below. Since the current version is 1.19 and Twirp promises to keep compatibility with the last 2 versions, the change seems fine. Can we confirm this?

[marwan-at-work]

@marioizquierdo Yes, the docs on the ioutil package say 1.16 as well https://pkg.go.dev/io/ioutil

[wmatveyenko]

Thank for your contribution. Can you add -

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[marwan-at-work]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[wmatveyenko]

Thanks for your contribution.

[marwan-at-work]

@wmatveyenko thank you (and to the others) for keeping the project maintained. Let me know if you plan on cutting a patch release. Thanks!