Terraform: update AKS version

[aLekSer]

Update AKS to 1.15.10.

What type of PR is this?

Uncomment only one /kind <> line, hit 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:

Which issue(s) this PR fixes:

Work on #1478

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 99b5c741-1dc4-47e5-8bb5-036aa83cba2a

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

• image: gcr.io/agones-images/agones-controller:1.6.0-5902cb8
• image: gcr.io/agones-images/agones-sdk:1.6.0-5902cb8
• image: gcr.io/agones-images/agones-ping:1.6.0-5902cb8
• Linux C++ SDK (build): agonessdk-1.6.0-5902cb8-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.6.0-5902cb8.zip

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

• https://5902cb8-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1556/head:pr_1556 && git checkout pr_1556
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.6.0-5902cb8

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aLekSer, pooneh-m

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