ci: Keep the CI Go version in the go toolchain group

GOVERSION picks the golang image for every Go job, but Renovate's built-in GitLab CI manager doesn't read variables:, so it fell behind the Dockerfile builder until a hand raise on 2026-10-05. This annotates it for the shared preset's annotated-variable manager (gitlab-com/public-sector/pipeline!195 (merged)), so the next Go release moves GOVERSION and the Dockerfile together in one go toolchain MR. In a local Renovate run both landed on renovate/go-toolchain.