Fix brainpool group and version downgrade handling for TLS 1.3, DTLS 1.3 and ECH - #11857

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/