Build is broken for tools

I merged #15373, not expecting us to have anything targeting old versions of .NET that were being removed. But in various places we target netstandard2.0, and I hadn't spotted that support for netstandard2.0 was removed at the same time. That feels like a bad idea to me, but I doubt that we'll get it rolled back. I *believe* we can just change the target for VersionCompat to be net8.0. I'm pretty sure we run all tools under .NET 8 anyway.