fix(oauth2-proxy.github.io): update Go build dependency - #14531

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: technote-space/get-diff-action@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/