Remove AcceleratorConnector.root_gpu and deprecate Trainer.root_gpu

[DuYicong515]

What does this PR do?

Part of #11449: remove unused properties in accelerator_connector: root_gpu
Part of #12171: deprecate device related properties in Trainer in favor of getting them from Trainer.accelerator or Trainer.strategy: root_gpu

AcceleratorConnector.root_gpu was not meant to be public. It's removed from AcceleratorConnector and deprecated in Trainer in favor of using Trainer.strategy.root_device.index if isinstance(Trainer.accelerator, GPUAccelerator) else None

Fixes #11994

Does your PR introduce any breaking changes? If yes, please list them.

N/A

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