Update JSON Ledger API OpenAPI reference

Updates the JSON Ledger API OpenAPI source pin to the latest public Canton release bundle for the published docs version and regenerates the checked-in OpenAPI reference.

Version changes:

• JSON Ledger API OpenAPI 3.5 canton_version: 3.5.15 -> 3.5.16

Validation run by the workflow:

• npm run update:generated-reference-sources -- --source ledger-api
• npm run generate:json-api-reference
• git diff --check