Docs: updated advised version of Kubernetes to use

[aLekSer]

Update supported version by Agones.

What type of PR is this?

/kind documentation

What this PR does / Why we need it:

Which issue(s) this PR fixes:

Relates to #1649

Special notes for your reviewer:

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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

The pull request process is described here

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• OWNERS [aLekSer,roberthbailey]

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[agones-bot]

Build Succeeded 👏

Build Id: e84b5a16-2ecb-4b52-a364-ff356336526a

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

• image: gcr.io/agones-images/agones-controller:1.9.0-87f0aae
• image: gcr.io/agones-images/agones-sdk:1.9.0-87f0aae
• image: gcr.io/agones-images/agones-ping:1.9.0-87f0aae
• Linux C++ SDK (build): agonessdk-1.9.0-87f0aae-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.9.0-87f0aae.zip

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

• https://87f0aae-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1789/head:pr_1789 && git checkout pr_1789
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.9.0-87f0aae