[codex] Update Go version and CI matrix

[sdcoffey]

Summary

• Bump the module minimum Go version from 1.13 to 1.21.
• Replace the legacy Travis configuration with the existing GitHub Actions Tests workflow.
• Run build and test jobs across Go 1.21, 1.22, 1.23, 1.24, 1.25, and 1.26.

Validation

• Parsed .github/workflows/test.yaml as YAML successfully.
• Confirmed the Tests GitHub Actions matrix passes for Go 1.21 through 1.26 on this PR.
• Could not run go test -v ./... locally because go is not installed in this environment.