[stubsabot] Bump tensorflow to ~=2.21.0

Release: https://pypi.org/pypi/tensorflow/2.21.0
Homepage: https://www.tensorflow.org/
Repository: https://github.com/tensorflow/tensorflow
Typeshed stubs: https://github.com/python/typeshed/tree/main/stubs/tensorflow
Diff: tensorflow/tensorflow@v2.18.1...v2.21.0

Stubsabot analysis of the diff between the two releases:
 - Total lines of Python code added: 32.
 - Total lines of Python code deleted: 150.

:warning: Review this PR manually, as stubtest is skipped in CI for tensorflow!
Also check whether stubtest can be reenabled. :warning:

Author: stubsabot

stubs/tensorflow/METADATA.toml

@@ -1,8 +1,8 @@
 # Using an exact number in the specifier for scripts/sync_protobuf/tensorflow.py
 # When updating, also re-run the script
-version = "~=2.18.0"
-upstream-repository = "https://github.com/tensorflow/tensorflow"
 # requires a version of numpy with a `py.typed` file
+version = "~=2.21.0"
+upstream-repository = "https://github.com/tensorflow/tensorflow"
 dependencies = ["numpy>=1.20", "types-protobuf", "types-requests"]
 extra-description = "Partially generated using [mypy-protobuf==3.6.0](https://github.com/nipunn1313/mypy-protobuf/tree/v3.6.0) and libprotoc 27.2 on `tensorflow==2.18.0`."
 partial-stub = true