Update to .NET 9 and Resolve FEC Version Compatibility Issues

[ismkdc]

Hi there,

First of all, thank you for all the great work on this project! We've been using Mapster and really appreciate the effort that's gone into it.

However, we've started facing some challenges recently. Specifically:

Compatibility issues with older dependencies: Upgrading the project to .NET 9 would help resolve some of these problems and keep things running smoothly.
FEC version errors: We've encountered issues while trying to use FEC versions 4.2.2 and the latest 5.0.0. These errors are making it hard for us to integrate them properly into our workflows.
We also noticed the project hasn’t received updates for about a year. Would it be possible to look into these issues and update the project? Your help would make a big difference to everyone relying on Mapster!

Thanks so much for your time and effort. Let me know if there’s anything I can do to assist. 😊
@andrerav

[andrerav]

.NET 9 fixed by #744. Let's keep the FEC discussion in #731.