Update TFMs #438

In version 9 we'll drop the old unsupported TFMs (net6.0, net7.0).

• Ardalis.Specification - net8.0;net9.0;netstandard2.0
• Ardalis.Specification.EntityFrameworkCore - net8.0;net9.0
• Ardalis.Specification.EntityFramework6 - net8.0;net9.0;net472;

For the base package, I'm keen to target net472 directly instead of netstandard2.0. But, I'm not quite sure if there are any users that still need netstandard specifically. Perhaps we'll remove it in version 10.
For the EF6 package, we'll also target net8.0 and net9.0 TFMs. Some of the users are still targeting EF6 in new projects. I assume there might be a subset of users who are still transitioning from netfx, so we'll keep supporting that.

For the EFCore package, even though we target both net8.0 and net9.0, we'll depend on the latest version of EF Core, 9.0.2 at the moment. The EF Core packages always depend on the latest LTS runtime, so EF Core 9 works on .NET 8.

PS. Technically, we don't even need to target the newest TFMs or the latest EF Core versions. We should specify only the minimum requirements. And that's what we were doing in the past. But, we're constantly getting complaints from users (there is a misunderstanding about how TFMs and dependencies work), so we'll multi-target all supported TFMs.