Bump Harmony Go toolchain and build images to Go 1.26.5

[Copilot]

This updates Harmony’s Go runtime/toolchain references to the latest stable Go release so local development, CI, and Dockerized builds use a consistent version. It removes drift between module metadata, workflow configuration, and container base images.

• Go module/toolchain alignment

  • Updated go.mod:
    • go 1.24.0 → go 1.26.0
    • toolchain go1.24.2 → toolchain go1.26.5

• CI runtime update

  • Updated PR workflow Go setup in .github/workflows/ci-pr.yaml:
    • go-version: '1.24.2' → go-version: '1.26.5'

• Container build/runtime update

  • Updated Docker base/toolchain versions across build images:
    • Dockerfile (GOLANG_VERSION)
    • scripts/macos_docker/Dockerfile (GOLANG_VERSION)
    • Test.Dockerfile (golang:1.26.5)
    • rosetta/infra/Dockerfile (golang:1.26.5)
    • api/service/synchronize/legacysync/downloader/Proto.Dockerfile (golang:1.26.5-bullseye)

• Docs consistency

  • Updated required Go version in README.md to Go 1.26.5.

- go 1.24.0
- toolchain go1.24.2
+ go 1.26.0
+ toolchain go1.26.5