Update all projects to .NET 9 and enable multi-targeting for libraries (.NET 8 and .NET 9)

[christiannagel]

Update all projects in the repository to target .NET 9. For library projects, enable multi-targeting so they support both .NET 8 and .NET 9.

Tasks:

• Update project files for applications to target .NET 9
• Update library projects to multi-target .NET 8 and .NET 9
• Update CI/CD pipeline definitions if necessary to test/build on .NET 9
• Update NuGet packages
• Verify compatibility and run all tests

This ensures forward compatibility and helps consumers transition smoothly to .NET 9 while maintaining support for .NET 8.