Test:  update test .NET SDK from .NET 7 preview to GA

[This PR hard-coded the .NET SDK version for testing](https://github.com/NuGet/NuGet.Client/pull/4791/files#diff-bd4ef9ba7459727e37706ab7197bdfaa3868ec01297f168ff0b6e243f087d239R46) because .NET 7 GA hadn't been released yet.

Now that .NET 7 GA has been released, there's no need to pin to a preview release of .NET 7.