Move runtime to Node 24, eliminate Node 20 deprecation warnings

[GeorgioCZY]

Summary

• Bumps the local JS action runtime from node20 → node24 (action.yml)
• Migrates all 7 slackapi/slack-github-action@v1.27.1 call sites to @v3.0.3, adopting the v2 input shape: method: chat.postMessage, token: input (replacing the SLACK_BOT_TOKEN env), and channel inside the JSON payload
• Bumps actions/setup-node@v4 → @v6 in the test workflow; raises the test runner from Node 20 → 24
• Regenerates dist/ via npm run prepare against Node 24

After this PR, the only Node 20 reference left is golangci/golangci-lint-action@v6 in go/lint/action.yml, which is the deliberate fallback branch for repos still on golangci-lint v1 config. No Node 24 release of that action supports v1 config — repos on v2 config already use @v9. Migrating consumers off v1 config is a separate effort.

Things to verify before merging

• The Slack v3 action cannot send to multiple channels in one step (v1 allowed comma-separated channel-id). The SLACK_CHANNEL_ID input descriptions still say "channel ID(s)" — if any downstream repo was passing multiple IDs in one call, that flow will now fail.
• dist/ shrunk by ~7k lines because the previously committed bundle was last rebuilt in Oct 2023; the source .js is unchanged. Tests pass against the rebuilt bundle.

Test plan

• CI green on this PR (units job runs npm test; test job exercises the composite action)
• Smoke-test a downstream Go repo against this branch's @chore/node24-deprecation-cleanup ref, watching for: Node 20 deprecation warning gone, Slack failure notifications still deliverable
• Confirm no downstream consumer relies on multi-channel SLACK_CHANNEL_ID

🤖 Generated with Claude Code