keras 2.16.0

• Updates for usage with Legacy Keras (#1515).

  • A package startup message is now displayed encouraging users to migrate to keras3.
  • New function py_require_legacy_keras().
  • install_keras() now installs legacy keras tf-keras.
  • TF_USE_LEGACY_KERAS=1 envvar is now set on package startup.

• Documentation updates for CRAN (#1514)