Remove usage of deprecated io/ioutil

[SuperSandro2000]

No description provided.

[matfax]

Deprecated: As of Go 1.16, the same functionality is now provided by package [io](https://pkg.go.dev/io) or package [os](https://pkg.go.dev/os), and those implementations should be preferred in new code. See the specific function documentation for details.

Source: https://pkg.go.dev/io/ioutil

[matfax]

I'm only guessing why this isn't merged yet. Either due to the empty body or because of the reformat in mapper.go.

[SuperSandro2000]

because of the reformat in mapper.go.

That is standard go fmt which got introduced in 1.20 or 1.21 for comment blocks. I can revert it but go will put it back when formatting.

[abhinav]

Superseded by #403.
Can probably be closed.