[janhartman]

Bumps this repository to Go 1.27.3, the latest stable Go release.

• Updates the go directive (and toolchain directive, if present) in every go.mod to 1.27.3 and runs go mod tidy where needed. No dependencies were intentionally upgraded; go.mod/go.sum changes come from tidy (e.g. the indirect requirements block required for Go 1.17+ modules).
• Updates other Go version pins (CI configs, golang Docker base images, tool version files) so the repository stays consistent.
• Floating tags such as golang:latest and Bazel toolchain versions are left unchanged.
• The prebuilt build-env CI images (build-env:1.17 / build-env:1.19) are left unchanged because no Go 1.27 build-env image is published yet. CI jobs that run in those containers need a rebuilt image from .github/Dockerfile.

Note: moving from a pre-1.22 go directive enables per-iteration loop variable semantics.

Created by a Sourcegraph agentic batch change.