[NET9] Trim warnings for ActivatableType

[lvde0]

ActivatableType was introduced to avoid trim warnings. I am targeting Kiota from a .NET 9 project and it seems to resolve the netstandard 2.1 version of the library due to lack of explicit .NET 9 support. This causes the trim warnings to reappear, as the fix is only available for NET5_0_OR_GREATER.

Correct me if I am wrong, but we should probably explicitly target net9.0 in all the libraries here?