Updates to xUnit v3 and enables Microsoft.Testing.Platform

[phil-scott-78]

Verify and Cake worked without a hitch. Very smooth upgrade process. Tests run too quick on my machine to see the fancy ANSI output for the runner, but it's there!

One neat thing about a change like this is that if the AOT route is tackled, theoretically it makes it much easier to test as we can run the tests in a PublishAOT scenario with a bit of leg work...hopefully.

There was a code change was responding to warnings regarding using TestContext.Current.CancellationToken . See this for the reason - https://xunit.net/xunit.analyzers/rules/xUnit1051