Dependency json-jwt allows bypass of identity checks via a sign/encryption confusion attack (CVE-2023-51774)

## Issue

The gem `rack-oauth2` has `json-jwt >= 1.11.0` as dependency which is vulnerable to CVE-2023-51774 (see https://github.com/advisories/GHSA-c8v6-786g-vjx6).

Patched versions are `1.16.6` and `1.15.3.1`.

## Temporary fix

Add `gem 'json-jwt', '>= 1.16.6'` to your gemfile to ensure the patched gem version.