Migrate codegen off deprecated jsonschema.RefResolver

[gsokoll]

jig/scripts/generate_node_interface.py imports RefResolver, deprecated since jsonschema 4.18 and slated for removal. Today it only emits a DeprecationWarning, but when a jsonschema release removes RefResolver the import fails and codegen breaks for every jig package — an unpinned jsonschema turns this into a hard build break.

Do:

1. Replace RefResolver with the referencing library (referencing.jsonschema), preserving the Draft7Validator behaviour.
2. Add a regression test asserting codegen runs warning-clean.
3. Interim guard for consumers: pin a jsonschema upper bound until (1) lands.

Ref: nineyards-robotics#8