chore(ci): bump actions to Node 24-ready versions and pin goreleaser

[daichirata]

Summary

The v0.5.0 release workflow run printed two warnings:

1. Node.js 20 deprecation. GitHub Actions removes Node.js 20 from the runner on 2026-09-16 and forces Node.js 24 as the default on 2026-06-02. The third-party actions in our workflows still ran on Node.js 20.
2. goreleaser `version: latest`. Floats with upstream; a future goreleaser major release could silently break the release pipeline.

Both addressed here.

Changes

Action	Before	After	Why
actions/checkout	v4	v6	v6.0.0+ uses `node24`
actions/setup-go	v5	v6	v6.0.0+ uses `node24`
docker/login-action	v3	v4	v4.0.0+ uses `node24`
goreleaser/goreleaser-action	v6	v7	v7.0.0+ uses `node24`
goreleaser binary (`with: version`)	`latest`	`'~> v2'`	Pin to current major

Each target version was verified by reading its `action.yml` and confirming `runs.using: node24`.

Test plan

• No code or behavior changes; workflow updates only
• CI green on this PR (test workflow exercises checkout/setup-go on the new majors)
• Next release (`v0.5.1` or beyond) verifies the release workflow chain end-to-end

🤖 Generated with Claude Code