[illink] Turn on serialization discovery in .NET 7

[sbomer]

dotnet/linker#2929 made serialization heuristics opt-in. To preserve the .NET 6 behavior, the Xamarin SDK should opt in to the old behavior by setting <_ExtraTrimmerArgs>--enable-serialization-discovery</_ExtraTrimmerArgs>. This can be done once using an SDK with dotnet/sdk#26993 (which will be part of rc1).

[rolfbjarne]

Fixed here: 62055c5