Build: .NET 6 SDK, drop ilrepack from build, change some target frameworks

[sandyarmstrong]

.NET Core 2.1 is not supported on arm64 Macs. .NET 6.0.300 SDK is a long-term support release with broad compatibility.

Newer Microsoft.Build package versions no longer have netstandard2.0 support (see https://github.com/dotnet/msbuild/issues/4521). Update Xamarin.MSBuild.Sdk to multi-target net6.0 and net472 to address this.

Xamarin.BuildConsolidator depends on ILRepack, which is not supported in .NET 6. The only actively-used consolidation support comes from PrepareConsolidationProject in Xamarin.MSBuild.Sdk, so remove the unused projects from the build.