FEATURE: Replace SLN with Updated SLNX format file
Detailed Description
After upgrading to at least .NET9 (#89) we should consider migrating the existing .SLN file to the newer simpler .SLNX version of the solution file.
https://devblogs.microsoft.com/visualstudio/new-simpler-solution-file-format/
One benefit of the newer format is that it should no longer be necessary to add project IDs (GUIDs) to the template.json file as the .SLNX file only needs the relevant .csproj file paths for referencing Projects within the Solution.
Just wondering if a .NET8 project could be migrated to this .slnx stuff. I know it got introduced with 9.0.200 SDK but I will have to try myself out if this works with a .NET8 project without having to upgrade to .NET9