Preparation for 1.12.0 release

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit 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

/kind release

What this PR does / Why we need it:
Updating all the versions.

Which issue(s) this PR fixes:

Work on #1936

Special notes for your reviewer:

N/A

[agones-bot]

Build Succeeded 👏

Build Id: f5b97b7c-5308-4dd7-b1f6-2116f7758883

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

• image: gcr.io/agones-images/agones-controller:1.12.0-0ebb75c
• image: gcr.io/agones-images/agones-sdk:1.12.0-0ebb75c
• image: gcr.io/agones-images/agones-ping:1.12.0-0ebb75c
• Linux C++ SDK (build): agonessdk-1.12.0-0ebb75c-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.12.0-0ebb75c.zip

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

• https://0ebb75c-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1938/head:pr_1938 && git checkout pr_1938
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.12.0-0ebb75c

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, pooneh-m, steven-supersolid

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• OWNERS [markmandel,pooneh-m]

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