Outdated curve id in example JWT config

[clehner]

The ES256K private key in config.json.jwt.example uses curve "P-256K", which is not in RFC 7518 or the JSON Web Key Elliptic Curve registry.

COSE and JOSE Registrations for WebAuthn Algorithms (draft) specifies "P-256K" for JWK. Later drafts changed it to "secp256k1", possibly to avoid confusion with "P-256" which is a different curve id. The draft was published as RFC 8812, entering "secp256k1" in the JWK Elliptic Curve registry.