Updated to 2021.3.21f1LTS

[dariomangoni]

Few GameObjects and settings were obsoleted in the latest versions of Unity, especially regarding the XR components.

The changes are intended to:

• update the repository to the latest LTS version
• expand the ReadMe to:
  • cover the latest Google Cardboard SDK
  • add other required XR Components (e.g. XR Origin, XR Interaction Manager)

No changes are made to the structure of the code, with the exception of the Assets/CardboardHelpers/Scripts/XRCardboardController.cs where the Camera.main was made read only and I preferred to just throw an error instead of automatic binding the object to the Main Camera automatically.

Please mind that I'm quite a newbie: the code works on my Android, but I didn't test on any other support.