Update linker defaults for .NET 6

[sbomer]

In dotnet/sdk#16327, the .NET SDK is changing the linker defaults to TrimMode=link, with SuppressTrimAnalysisWarnings=false. Xamarin-macios should consider doing the following when this change flows:

• Default SuppressTrimAnalysisWarnings to true to avoid producing warnings
• Avoid setting TrimMode in https://github.com/xamarin/xamarin-macios/blob/32e4d05195abe86c5a02ed38a0abd232cd5f47aa/dotnet/targets/Xamarin.Shared.Sdk.targets#L269