Base64 encode and decode: retarget net10.0, fix the Tests.cs encoding defect, add files, streams, TryDecode and Base64Url - #2173

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/