Upgrade TensorFlow to 2.21.0 and fix build dependencies

[rtg0795]

1. TensorFlow Upgrade to v2.21.0 & Toolchain Adjustments

   • Upgraded the @org_tensorflow dependency from 2.20.0 to 2.21.0.
   • Reordered repository initialization in WORKSPACE so tf_workspace3 runs before hermetic Python initialization (required by TF 2.21+ to define @xla and @tsl).
   • Dropped support for Python 3.9 (removed requirements_lock_3_9.txt).
   • Consolidated multiple separate pywrap dependencies under a unified //tensorflow_text/core/pybinds:pybinds_library target.
   • Applied necessary compilation patches to various TensorFlow subsystems (framework, common runtime, grappler optimizers, device set, etc.).
   • Updated @local_xla references to @xla.

2. Kernel Test Dependency Cleanup

   • Removed redundant _for_test wrapper libraries in tensorflow_text/core/kernels/BUILD (such as constrained_sequence_op_for_test and ngrams_op_for_test).
   • Updated C++ unit tests to link directly against standard op C++ libraries (e.g., //tensorflow_text:constrained_sequence_op_cc), eliminating duplicate global initializer linkage conflicts.

3. Pip/Configure Script Fixes

   • Explicitly imported compile_pip_requirements from @rules_python//python:pip.bzl in oss_scripts/pip_package/BUILD to resolve dependency resolution errors during configuration.