Patch release: bump tested TensorFlow / Keras versions to the latest.
- Bump CI pin and the README's documented tested versions to TensorFlow 2.21.0 / Keras 3.14.0 (was 2.18.0 / 3.8.0).
- Accept the renamed Threshold layer config keys Keras 3.14 emits (`threshold` and `default_value`), with fallback to the old keys (`threshold_value` and `value`) for models exported by older Keras versions.
- Update the perf-test Dockerfile to match: bazel-7.7.0 (required by TF 2.21's `.bazelversion`), `xxd` tooling, and an explicit `keras==3.14.0` pin.
Verified by running the test suite on the new versions and a full end-to-end docker build of the perf-test Dockerfile.