OpenFGA Playground Preshared Key Exposure

OpenFGA is an authorization/permission engine built for developers. In versions 0.1.4 through 1.13.1, when OpenFGA is configured to use preshared-key authentication with the built-in playground enabled, the local server includes the preshared API key in the HTML response of the /playground endpoint. The /playground endpoint is enabled by default and does not require authentication. It is intended for local development and debugging and is not designed to be exposed to production environments. Only those who run OpenFGA with `--authn-method` preshared, with the playground enabled, and with the playground endpoint accessible beyond localhost or trusted networks are vulnerable. To remediate the issue, users should upgrade to OpenFGA v1.14.0, or disable the playground by running `./openfga run --playground-enabled=false.`