Migrate solution to slnx format (#1749)

see https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/

Author: mus65

.gitattributes

@@ -2,7 +2,6 @@
 
 *.cs text diff=csharp
 *.xaml text
-*.sln eol=crlf
 *.csproj eol=crlf
 *.shproj eol=crlf
 *.appxmanifest eol=crlf

.github/workflows/build.yml

@@ -65,7 +65,7 @@ jobs:
       uses: actions/setup-dotnet@v5
 
     - name: Build Solution
-      run: dotnet build Renci.SshNet.sln
+      run: dotnet build Renci.SshNet.slnx
 
     - name: Publish AOT Compatibility Test App
       run: dotnet publish -r win-x64 /warnaserror test/Renci.SshNet.AotCompatibilityTestApp/