Examples: update Kubernetes version

[aLekSer]

Update YAML and md files: website updated.

What type of PR is this?

/kind documentation

What this PR does / Why we need it:
Update website to point to a specific Kubernetes version advised to use with Agones.

Which issue(s) this PR fixes:
Relates to #1649

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 25ad4cd5-41ee-4f76-b9ac-d7ebec90a574

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-b4a3ec6
• image: gcr.io/agones-images/agones-sdk:1.9.0-b4a3ec6
• image: gcr.io/agones-images/agones-ping:1.9.0-b4a3ec6
• Linux C++ SDK (build): agonessdk-1.9.0-b4a3ec6-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.9.0-b4a3ec6.zip

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

• https://b4a3ec6-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1787/head:pr_1787 && git checkout pr_1787
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.9.0-b4a3ec6

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [roberthbailey]

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