Fix langversion typo in union types tutorial - #55774

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