Powershell: Update PackageReferences to GA releases

[SteveL-MSFT]

As part of PR https://github.com/PowerShell/PowerShell/pull/6958 we updated dotnet-xunit to 2.4.0-beta.1.build3958 and xunit to 2.4.0-beta.2.build4010. Microsoft.VisualStudio.Web.CodeGeneration.Tools is also still using a non-GA release version.

[bergmeister]

I was briefly looking at this and it seems the current status is:

• [ ] [xunit](https://www.nuget.org/packages/xunit): 2.4.0-beta.2.build4010 (2.4.0-rc.1.build4038 would already be available)
• [ ] [Microsoft.VisualStudio.Web.CodeGeneration.Tools](https://www.nuget.org/packages/Microsoft.VisualStudio.Web.CodeGeneration.Tools): 2.1.0-preview1-final
• [ ] [dotnet-xunit](https://www.nuget.org/packages/dotnet-xunit): 2.3.1 (in tests/hosting) and 2.4.0-beta.1.build3958 (in tests/xunit`) in use

[SteveL-MSFT]

The xunit assemblies are not GA blocking for PSCore6.1 since it only affects the test code. We should move to the final build of CodeGeneration, however.

[iSazonov]

@SteveL-MSFT We can close the Issue - package versions was updated and now a bot does these updates.