Add SE5Plugins.slnx solution for se5 plugin projects - #290

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, actions/upload-artifact@v4, microsoft/setup-msbuild@v1, nuget/setup-nuget@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/