Updating GenAPI to properly handle nint for C# 9 and later

[tannergooding]

CC. @ViktorHofer, @ericstj

[tannergooding]

Prior to C# 11, IntPtr was IntPtr and nint was [NativeInteger] IntPtr.

C# 11 and later, its just nint provided at least .NET 7 is targeted. Since C# 11 is only "officially" supported on .NET 7 and later, GenAPI can just treat this "synonymously".

[tannergooding]

dotnet/runtime#69627 is the runtime side change that picks up the compiler changes

[ViktorHofer]

I kindly ask @ericstj to review this change, as it touches the CCI codebase.

[ericstj]

I'd like someone from @dotnet/area-infrastructure-libraries review the code for correctness and provide signoff.

Consider testing methods like those previously suggested in #8769 (comment) and share the details for how you tested this to ensure the changes work as expected without regressions.

[tannergooding]

@ericstj, I validated this against dotnet/runtime as always.

The change is simply that on .NET 7 and later nint should always be used (IntPtr and nint are aliases of each other and we prefer keywords over fully qualified type names). Prior to .NET 7 its used if the relevant attribute is present.

[tannergooding]

Ping @dotnet/area-infrastructure-libraries could this get a review and merge?

[tannergooding]

@ViktorHofer could I get a sign-off on this one as well?

[ViktorHofer]

I asked @carlossanlop to take a look here as he recently touched the cci code base as well. I myself never worked on that library so I would like to defer to Carlos.

[ViktorHofer]

Based on @tannergooding's comment, the reference sources in dotnet/runtime are already generated based on these changes so we should get this in asap. Merging.