ci: remove Node.js 16 from CI matrix (EOL Sep 2023)

## Summary

The `node.js.yml` `build` job currently runs a matrix across Node.js `[16, 20]`. Node.js 16 reached end-of-life in **September 2023** and should be removed.

The `lint` and `test` jobs already use `[20]` only — this only affects `build`.

## Change

**File:** `.github/workflows/node.js.yml`

```yaml
# Before
strategy:
matrix:
node-version: [16, 20]

# After
strategy:
matrix:
node-version: [20]
```

## Scope

`node.js.yml` is explicitly excluded from the `sync-workflows.yml` sync (to preserve per-repo benchmark customisations). After merging the canonical change here, all 35 downstream `frmscoe/rule-*` repos must be updated individually via script (see implementation plan in the comments).

## Acceptance Criteria

- [ ] `frmscoe/workflows` canonical `node.js.yml` has `node-version: [20]` only in the `build` job
- [ ] All active `frmscoe` rule repos (rule-001 through rule-091) have `node-version: [20]` only in `node.js.yml`

## Related

- Parallel track: [tazama-lf/workflows#23](https://github.com/tazama-lf/workflows/issues/23) (Track A — additional changes for tazama-lf service repos)