Benchmarks: Run EF Core dev benchmarks with netcoreapp2.1 runtime

[smitpatel]

Also upgrade versions of EF to latest on nuget for older versions.

cc: @divega

[ajcvickers]

All the 1.x stuff can go.

We should have:

• 2.1 EF Core on 2.1 .NET Core
• 2.0 EF Core on 2.0 NET Core
• 2.1 EF Core on 2.0 NET Core
• 2.1 EF Core on .NET Framework
• 2.0 EF Core on .NET Framework
• EF6 on .NET Framework