Move flowSecret to a request header for /flow/execute - #3808

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, errata-ai/vale-action@d89dee975228ae261d22c15adcd03578634d429c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/