Update Benchers for EF6, EF Core and PetaPoco

[jonnybee]

Update .NET project s to .NET 4.8 and NET Core projects to .NET5

Update benchers for EF6 and EF Core so that:

• EF6 benchers run in both .NET 4.8 and NET5
• Update EF Core benchers to use EF Core 3.1 in .NET4.8 and EF Core 5 in .NET5
• Update EF Core DTO bencher to run in Both .NET4.8 and .NET5
• Update PetaPoco benchers to use PetaPoco.Compiled v6

[FransBouma]

Implemented in #56