Upgrade to .NET 9

[wendellmva]

Summary

Upgrade all projects from .NET Core 3.1 / .NET Standard 2.0 to .NET 9.

Tasks

• Update src/Directory.Build.props with new package versions
• Change all .csproj target frameworks to net9.0
• Update LangVersion to latest
• Upgrade Microsoft.EntityFrameworkCore to 9.0.0
• Upgrade Microsoft.Extensions packages to 9.0.0
• Upgrade NuGet packages to 6.8.0
• Update test project target frameworks
• Fix any breaking changes in code
• Update CI/CD workflow to use .NET 9
• Update Dockerfile base images to 9.0

Files to Modify

• src/Directory.Build.props
• All .csproj files in src/ and tests/
• .github/workflows/main.yml
• Dockerfile

Acceptance Criteria

• dotnet build succeeds
• dotnet test passes
• Docker image builds successfully