Update the Kubernetes upgrade instructions to include instructions for upgrading gRPC

[roberthbailey]

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:
Follow up to #2725 and #2727 so that the next time we do a Kubernetes upgrade we remember how to upgrade gRPC across the project.

I also snuck in a documentation fix to include make gen-all-sdk-grpc in the build documentation since it was previously missing.

Which issue(s) this PR fixes:

n/a

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 06fabc64-a3ce-4309-a0c7-487feea8db20

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:1.27.0-6dce7d5-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.27.0-6dce7d5-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.27.0-6dce7d5-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.27.0-6dce7d5-amd64
• Linux C++ SDK (build): agonessdk-1.27.0-6dce7d5-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.27.0-6dce7d5-amd64.zip

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

• https://6dce7d5-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2755/head:pr_2755 && git checkout pr_2755
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.tag=1.27.0-6dce7d5-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [markmandel,roberthbailey]

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