Update demo to run under 2021.3 (LTS)

Now that the UI Toolkit is (as I understand it) included with 2021.3, not as a preview package and with runtime support, can you update the demo project?

https://github.com/FreeUnityProjects/UIToolkitUnityRoyaleRuntimeDemo

I just want to do a git clone..., then open the project in 2021.3 and have it work, without having to fix anything, install any secret github packages, convert the rendering material, etc.

I'm quite sure that the new UI Toolkit is much better than using...

Update demo to run under 2021.3 (LTS)