Migrate from the deprecated io/ioutil package

[reugn]

Motivation

The io/ioutil package has been deprecated in go1.16, see the release notes. This PR replaces the existing io/ioutil calls with their new definitions in the io and os packages.

Verifying this change

• Make sure that the change passes the CI checks.

This change is already covered by existing tests.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

• Does this pull request introduce a new feature? (yes / no)

[nodece]

For me, we should always use the supported version from the golang team.

[pgier]

LGTM, although I wonder if we should cut one more release before breaking compatibility with golang 1.15

[nodece]

We can use a build flag to be compatible with the different versions.

[pgier]

@nodece That's certainly possible, but in my opinion not worth the extra maintenance effort. It would mean that any bug fixes and new features would also need to be tested against the older versions. At some point we should just tell users to upgrade their Go version, especially since golang itself only maintains the two most recent versions.

My suggestion is to just cut one more release and update the docs to indicate that is the last version which works on go 1.15.