Fix GKE Autopilot auto-detection for 1.26

[zmerlynn]

The workload-defaulter no longer exists as a standalone webhook on 1.26 - instead use the new warden-mutating webhook. I am following up internally with finding a less brittle way to do this.

[agones-bot]

Build Succeeded 👏

Build Id: d26eced0-4410-4b8d-8bc0-c2fdb4e5e554

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:1.31.0-8ccf940-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.31.0-8ccf940-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.31.0-8ccf940-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.31.0-8ccf940-amd64
• Linux C++ SDK (build): agonessdk-1.31.0-8ccf940-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.31.0-8ccf940-amd64.zip

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

• https://8ccf940-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3032/head:pr_3032 && git checkout pr_3032
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.31.0-8ccf940-amd64

[gongmax]

nit: do we have unit test for this logic?

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: gongmax, zmerlynn
Once this PR has been reviewed and has the lgtm label, please assign ericfortin for approval. For more information see the Kubernetes Code Review Process.

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

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment