Migrate solution from .sln to .slnx format

[ctolkien]

This PR migrates the solution file from the legacy .sln format to the new .slnx format introduced in recent versions of .NET.

Changes Made

• ✅ Removed the legacy Slugify.sln file
• ✅ Added new Slugify.slnx file using dotnet sln migrate command
• ✅ Verified that all project references and configurations are maintained
• ✅ Confirmed GitHub Actions workflows are compatible with the new format

Benefits

• Modern Format: Uses the new XML-based solution format
• Better Performance: Faster loading and processing by .NET tools
• Enhanced Tooling: Better support in modern IDEs and editors
• Future-Proof: Aligns with Microsoft's direction for .NET solutions

Testing

The migration preserves all existing functionality:

• All projects remain properly referenced
• Build configurations are maintained
• Solution folders structure is preserved
• Existing CI/CD pipelines continue to work without modification

The dotnet CLI commands in GitHub Actions automatically detect and use .slnx files, so no workflow changes are required.

[ctolkien]

On hold until .NET10