Update 1.21.1 in version.py and RELEASE.md

[vkarampudi]

Align protobuf dependency with compiled version and bump to 1.21.1

Description

This PR resolves a critical dependency mismatch introduced in the v1.21.0 release (Issue #68).

In v1.21.0, TFMD's build configuration (WORKSPACE) was upgraded to compile all C++ extensions against protobuf 6.31.1. However, setup.py still permitted older runtime versions of the protobuf package (e.g., protobuf>=4.25.2). When users installed the library, pip could resolve to an older protobuf runtime package, causing immediate startup and runtime crashes due to Protobuf C++ ABI/version mismatch checks (e.g., google::protobuf::internal::VerifyVersion(...) aborting the process).

To fix this, this PR:

1. Global Protobuf Requirement Alignment: Sets the protobuf package dependency in setup.py to protobuf>=6.31.1,<7 globally across all supported Python versions (>= 3.10).
   • Why >=6.31.1: Since the C++ extensions in the wheels are compiled against 6.31.1, the runtime package version must be at least 6.31.1 to pass Protobuf's strict runtime verification checks and avoid process crashes.
   • Why <7: Adds an upper bound to prevent breaking ABI changes from future major protobuf releases.
   • Cleaned up Environment Markers: Completely removes the out-of-date conditional Python-version-specific requirements for protobuf, making the configuration much cleaner.
2. Release Preparation:
   • Bumps version to 1.21.1 in tensorflow_metadata/version.py.
   • Adds the release notes for 1.21.1 in RELEASE.md following the standard section structure.

---

Changes

• setup.py: Simplified and updated the global protobuf requirement to protobuf>=6.31.1,<7.
• tensorflow_metadata/version.py: Bumped __version__ from "1.21.0" to "1.21.1".
• RELEASE.md: Added release notes and standard sections for v1.21.1.

---

Verification & Testing

1. Local Wheel Building: Verified that python setup.py bdist_wheel builds and packages the library successfully with the new version and dependencies.
2. Pre-commit Checks: Ran pre-commit run --all-files locally; all linting, formatting (ruff), and sanity checks passed.
3. Python Test Suite: Ran the python test suite via pytest (python -m pytest), confirming that all tests (including dynamic proto loading) pass perfectly.
4. Ecosystem Compatibility Verification:
   • TensorFlow 2.21.0: Verified via pip install dry-run that tensorflow==2.21.0 and our local tensorflow-metadata==1.21.1 wheel resolve perfectly together without conflicts.
   • TensorFlow 2.18.0 (Older series): Confirmed that pip correctly flags a conflict as expected (since older TF versions use the previous protobuf epoch), preventing runtime ABI crashes for users.