Proj5000: Use SLNX solution files

Solution files have been a part of the .NET and Visual Studio experience for many years now, and they’ve had the same custom SLN format the whole time. Starting in .NET SDK 9.0.200, the dotnet CLI supports the use of SLNX format, a less verbose, easy to read XML format that resembles the other MSBuild (project) file structure.

If upgrading your build to use at least the .NET SDK 9.0 is not possible, we recommend to disable this rule, or reduce its severity for the time being. Information on how to that can be found here.

The latest SDK can be downloaded at dotnet.microsoft.com/download.

Further read: Microsoft’s blog.