Upgrade terraform to Kubernetes 1.20.

[roberthbailey]

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: First step of upgrading Kubernetes support to 1.20.

Which issue(s) this PR fixes:

Work on #2186

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 826d588e-67b0-45df-9e5a-11e5929bc5e7

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

• image: gcr.io/agones-images/agones-controller:1.17.0-73167af
• image: gcr.io/agones-images/agones-ping:1.17.0-73167af
• Linux C++ SDK (build): agonessdk-1.17.0-73167af-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.17.0-73167af.zip

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

• https://73167af-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2187/head:pr_2187 && git checkout pr_2187
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.17.0-73167af

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [markmandel,roberthbailey]

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