feat: AR migration for kokoro, pt 1

[muncus]

Description

Update kokoro image builds, and the latest-version kokoro build to use Artifact
Registry.

The container images exist for go 1.21 and 1.23 already.

Part of #4228

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[iennae]

approved, but kokoro CI is failing with minimal output so this might not be an ok change.

[muncus]

Looks like something else is needed to allow kokoro to pull images from the new location.

[11:16:59 PDT] ERROR: This is a presubmit run, but Kokoro always reads job configs from head. If your un-submitted change modifies allowed_env_vars, it is ignored.
[11:16:59 PDT][ERROR] Build failed due to an error
com.google.devtools.kokoro.controller.UserException: Found un-allowed variables:

TRAMPOLINE_IMAGE="us-docker.pkg.dev/golang-samples-tests/kokoro-images/go123" (No matching value in allowed env vars)

[muncus]

the failing tests on this run are caused by quota and "already exists" errors, neither of which appear to be caused by this change.