Replace uses of the io/ioutil package

[gongmax]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:
"io/ioutil" has been deprecated since Go 1.16, this PR made the necessary change to remove its usage
Which issue(s) this PR fixes:

Closes #2748

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 233f8961-4c3e-4b77-aac7-eda5a5f18860

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.27.0-518948a-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.27.0-518948a-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.27.0-518948a-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.27.0-518948a-amd64
• Linux C++ SDK (build): agonessdk-1.27.0-518948a-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.27.0-518948a-amd64.zip

A preview of the website (the last 30 builds are retained):

• https://518948a-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2752/head:pr_2752 && git checkout pr_2752
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.tag=1.27.0-518948a-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gongmax, roberthbailey

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [roberthbailey]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment