Audit TargetFramework usage for .NET 5 TFM change

[sfoslund]

With .NET 5 we are changing target framework from the netcoreappX.X style to netX.X. With this change, it appears this repo has code that could possibly break. Please check out https://github.com/dotnet/fsharp/blob/main/FSharp.Profiles.props#L10 to ensure that this logic works as intended for .NET 5 TFMs, noting that $(TargetFramework.StartsWith('netcoreapp')) will no longer be true with net5.0, thanks!