Deprecate and remove old QAT API

[andrewor14]

Following #2629, we plan to change the main QAT API (more details in the PR description). Today we keep around the following classes for BC without any warning:

# Replaced by QATConfig
IntXQuantizationAwareTrainingConfig
FromIntXQuantizationAwareTrainingConfig
intx_quantization_aware_training_config
from_intx_quantization_aware_training_config

# Replaced by IntxFakeQuantizeConfig
FakeQuantizeConfig

# Replaced by IntxFakeQuantizer
FakeQuantizer

The plan is to deprecate these configs before v0.13.0, migrate all known users, and remove these configs before v0.14.0. All references to the old API in the docs and README should already be made in #2629.