Update GKE installation instructions now that SplitControllerAndExtensions has been enabled by default.

[roberthbailey]

What type of PR is this?

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/kind bug
/kind cleanup

/kind documentation

/kind feature
/kind hotfix

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Which issue(s) this PR fixes:

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[agones-bot]

Build Succeeded 👏

Build Id: 862f493b-0e33-466a-ada4-fa712006f09b

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:1.33.0-5a2105a-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.33.0-5a2105a-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.33.0-5a2105a-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.33.0-5a2105a-amd64
• Linux C++ SDK (build): agonessdk-1.33.0-5a2105a-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.33.0-5a2105a-amd64.zip

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

• https://5a2105a-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3191/head:pr_3191 && git checkout pr_3191
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.33.0-5a2105a-amd64

[agones-bot]

Build Failed 😱

Build Id: eeb7b0e5-f281-4ba2-ba05-a2fc3a84708c

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[agones-bot]

Build Succeeded 👏

Build Id: b9455805-d38f-4073-bbcb-c762b5f0a6a7

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:1.33.0-e22f969-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.33.0-e22f969-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.33.0-e22f969-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.33.0-e22f969-amd64
• Linux C++ SDK (build): agonessdk-1.33.0-e22f969-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.33.0-e22f969-amd64.zip

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

• https://e22f969-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3191/head:pr_3191 && git checkout pr_3191
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.33.0-e22f969-amd64