[+] bump Go to 1.27

[pashagolub]

What

• go.mod: go 1.27.0
• CI workflows (build.yml, release.yml): actions/setup-go now reads the version from go.mod via go-version-file, so future bumps only touch go.mod
• release.yml: checkout moved before Go setup so go.mod is available
• build.yml: golang:1.27 Docker build image (image tags can't be derived from go.mod, still manual)

Why

Move the whole project to Go 1.27 and keep the module version, CI build and release toolchain in sync from a single source.

Notes for reviewers

• No other files pin a Go version (no Dockerfiles, docs or toolchain directive).
• Verified locally with go1.27.0: go build ./... and go vet ./... pass.

[coveralls]

coverage: 91.434% (-0.7%) from 92.15% — claude/go-1-27-upgrade-021fb7 into master