Deprecate optax.inject_stateful_hyperparameters and replace it with optax.inject_hyperparameters

[copybara-service[bot]]

Deprecate optax.inject_stateful_hyperparameters and replace it with optax.inject_hyperparameters

Other minor improvements along the way:

• add ..versionadded and ..deprecated tags to the docstrings for added parameters
• test docstring in inject_hyperparams (and catch a bug along the way)
• remove unused inject target from BUILD