Group an aggregating item over a property key by the property, and refuse a key of another shape there, as drivine4j does, at Cypher spec v0.14.0 - #315

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

The Cypher spec is at v0.16.0; drivine-python runs v0.14.0. Move to it with scripts/sync_cypher_spec.py v0.16.0.