[Trimming] Consider using standard <IsTrimmable> property

[JamesNK]

Currently ASP.NET Core has a Trimmable property to indicate trimming. Custom MSBuild configuration then adds the [AssemblyMetadata("IsTrimmable", "True")] attribute to assemblies.

Consider moving to the standard IsTrimmable property that is built into .NET 6+ - https://docs.microsoft.com/en-us/dotnet/core/deploying/trimming/prepare-libraries-for-trimming#set-istrimmable.

IsTrimmable should continue to drive the generation of known trimmable assemblies which are passed to the LinkabilityChecker.

[JamesNK]

TrimmingAnalyzers is automatically enabled with IsTrimmable. Unfortunately, there are errors when it runs on the aspnetcore code base. Analyzers have been disabled as a workaround

Rename trimming analyzers once dotnet/linker#2718 is fixed and the changes have flowed to the aspnetcore repo.