Make the C# language check a warning instead of an error

[kzu]

Currently we report an error (both on BeforeCompile and CoreCompile) when we detect the project language isn't C# 9.0+.

This breaks a few IDE experiences since right after package installation, all design-time builds and real builds are effectively broken, which isn't a great uex.

By making it a warning instead, we give the user the chance to build, see it, perhaps attempt to type ThisAssembly and see that nothing happens, and then cleanly uninstall the package.