fix CI: upgrade Node.js from 17 to 20

[AmSach]

Summary

Upgrade Node.js version in GitHub Actions from 17 to 20.

Problem

pnpm latest requires Node.js >= 18.12 but the workflow was using Node 17, causing CI failures:

ERROR: This version of pnpm requires at least Node.js v18.12
The current version of Node.js is v17.9.1

Solution

Update node-version from "17" to "20" in the setup-node action.

Testing

CI should pass after this change.

[AmSach]

The CI is failing because the workflow uses Node.js 17 but pnpm 11+ requires Node.js v22+. Here's the fix for .github/workflows/test.yml:

name: Check formatting
on:
  push:
  pull_request:

jobs:
  check:
    name: Check code formatting and run tests
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: "20"
      - uses: pnpm/action-setup@v4
        with:
          version: latest
          run_install: true
      - name: Check formatting
        run: pnpm run check-formatting

Key changes:

• node-version: "17" → "20" (pnpm 11 requires Node 22+, so Node 20 is the minimum stable choice)
• actions/checkout@v2 → v4
• actions/setup-node@v2 → v4
• pnpm/action-setup@v2 → v4

[AmSach]

Correction: the CI failure shows pnpm 11 requires Node.js v22.13, not v18.12. Node 20 alone won't work.

The fix needs either:

1. Node 22+: Set node-version: "22" (or "23"), OR
2. Pin pnpm to v9: Set version: "9" in pnpm/action-setup@v4 (pnpm v9 works with Node 20)

Option 1 is cleaner since Node 22 is LTS and future-proof.

[radiantly]

See #59 (comment)