Update README to use built-in browser methods instead of the deprecated `@github/webauthn-json` library

[Petersoj]

The @github/webauthn-json library was recently deprecated:

As of March 2025, stable versions of all major browsers now support the following methods:

• PublicKeyCredential.parseCreationOptionsFromJSON(…)
• PublicKeyCredential.parseRequestOptionsFromJSON(…)
• PublicKeyCredential → .toJSON()

These should be used instead of @github/webauthn-json.