slapconfiguration: support jsonschema >= 4.18 via the referencing library

jsonschema 4.18 deprecated the in-tree RefResolver and routes cross-file $ref resolution through the new referencing library. It also inlined Validator.descend so it no longer goes through iter_errors, which breaks the recipe's collect-defaults / unstringify scheme that relies on seeing every descended sub-schema. Without this fix bumping jsonschema above 4.17.x causes 40 errors + 18 silent failures in the JsonSchema* test classes (PointerToNowhere on cross-file $ref, plus defaults and unstringification silently noop'ing).

The fix is gated on a runtime detection of referencing so older Python 2 / jsonschema 3.0.2 environments keep working unchanged:

• Build a referencing.Registry(retrieve=...) driven by SoftwareReleaseSchema._readAsJson to lazily load sibling schema files; pass it via a new registry= kwarg on DefaultValidator and forward it to the validator constructor in validate().
• Add helpers (_capture_scope, _apply_scope, _resolver_of) that switch between validator.resolver.base_uri / push_scope (RefResolver) and validator._resolver / evolve(_resolver=...) (referencing.Resolver). Use them in iter_errors, memoize_defaults, collect_defaults, apply_unstringify, fetch_key.
• Re-route kls.descend through evolved.iter_errors(instance) on jsonschema >= 4.18 so the recipe's iter_errors wrapper observes every sub-schema descent again.

stack/slapos.cfg: bump jsonschema 4.17.3 -> 4.23.0 and pin its new transitive deps (referencing, jsonschema-specifications, rpds-py) so the stack can carry the new resolver chain. Wheel-only because rpds-py builds with maturin and referencing / jsonschema-specifications use hatchling.