Proj5001: Remove SLN 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.

When using SLNX solution files, there is no need to also keep SLN solution files. This rule helps to track those files that still exist on disk.