Update Rust SDK build image

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:

Looks like some libraries that this depends on require a more up-to-date Rust language release.

Which issue(s) this PR fixes:

Unblocks CI.

Special notes for your reviewer:

N/A

[agones-bot]

Build Succeeded 👏

Build Id: 43527520-4693-4251-86f6-af56591c7e15

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

• image: gcr.io/agones-images/agones-controller:1.24.0-3f0cdd6-amd64
• image: gcr.io/agones-images/agones-controller:1.24.0-3f0cdd6-arm64
• image: gcr.io/agones-images/agones-ping:1.24.0-3f0cdd6-amd64
• image: gcr.io/agones-images/agones-ping:1.24.0-3f0cdd6-arm64
• Linux C++ SDK (build): agonessdk-1.24.0-3f0cdd6-arm64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.24.0-3f0cdd6-arm64.zip

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

• https://3f0cdd6-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2635/head:pr_2635 && git checkout pr_2635
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.tag=1.24.0-3f0cdd6-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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Needs approval from an approver in each of these files:

• OWNERS [markmandel,roberthbailey]

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