Introduce support for .NET 9

[SonnyRR]

Add support for the .net 9 TFM, since currently projects which target this version are unable to generate models & mappers with the .net 8 pre-release version.

[ismkdc]

Thank you for your effort, but the latest versions(>=4.2.2) of FEC is still throwing errors.
#731

[andrerav]

Fixed by #744