Convert `.sln` files to `.slnx`

[xtqqczze]

The .NET team recently announced support for .slnx, a new, simplified solution file format for the .NET CLI:
Introducing support for SLNX, a new, simpler solution file format in the .NET CLI.

Draft changes from automated conversion using dotnet sln migrate: xtqqczze@1d7529a

[CSumme10]

See pull request #26465. I changed the files that were mentioned here in the issue from .sln to .slnx files. I also changed any mention of .sln to .slnx throughout the code.