update oai and free5gc exercises - #307

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332, errata-ai/vale-action@91ac403e8d26f5aa1b3feaa86ca63065936a85b6, step-security/harden-runner@91182cccc01eb5e619899d80e4e971d6181294a7. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/