Migrate xUnit test project templates to xUnit v3

`dotnet new xunit` and the "xUnit Test Project" template still create a xUnit v2 project. The xUnit v2 NuGet packages have all been marked as deprecated. It is a bad experience for a developer to get a deprecated package reference in a newly created project. So the xUnit test template should be updated to xUnit v3.

Note that xUnit v3 now has it's own template, see `dotnet new install xunit.v3.templates` and https://github.com/xunit/xunit/tree/main/src/xunit.v3.templates So maybe it would instead make sense to remove the xUnit template in this repository and instead install the template provided by xUnit by default?

Start with the `dotnet new xunit` command and the "xUnit Test Project" template, then compare them with the templates from `xunit.v3.templates`. Determine whether this repository should update its generated package references or install the upstream template by default; done means new xUnit projects no longer reference deprecated v2 packages.

Written by the indexing model from the issue text.