Update BenchmarkDotNet and remove net5.0 workaround

Workaround so BDN works in net5.0 - https://github.com/dotnet/aspnetcore/pull/23000

When [this fix](https://github.com/dotnet/BenchmarkDotNet/pull/1479) is merged and a new BDN is on NuGet we can upgrade and remove the workaround.