[tonywangcn]

What changed

• Upgrade the Go directive and pinned builder image to Go 1.27.3 to resolve GO-2026-6809 in crypto/x509.
• Keep the echo-probe fixture response behind the asynchronous request write so the upload counter is updated before the test checks traffic totals.
• Update the documented minimum Go version.

Validation

• make check
• Echo proxy test repeated 100 times
• Staticcheck
• Govulncheck: no vulnerabilities found
• make kustomize
• git diff --check