Bumps simple-game-server version to 0.18

[igooch]

What type of PR is this?

/kind cleanup

What this PR does / Why we need it:

Updates the simple-game-server tag references to the newest version 0.18 updated in #3378.

Which issue(s) this PR fixes:

Working on #2716

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 17b7b03c-3934-4df7-980f-1a53bc2bd08f

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:1.35.0-dev-6b9b2f8-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.35.0-dev-6b9b2f8-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.35.0-dev-6b9b2f8-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.35.0-dev-6b9b2f8-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.35.0-dev-6b9b2f8-amd64
• Linux C++ SDK (build): agonessdk-1.35.0-dev-6b9b2f8-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.35.0-dev-6b9b2f8-amd64.zip

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

• https://6b9b2f8-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3379/head:pr_3379 && git checkout pr_3379
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.35.0-dev-6b9b2f8-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [roberthbailey]

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