build(deps-dev): bump tensorflow from 2.18.0 to 2.21.0

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.18.0 to 2.21.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.18.0...v2.21.0)

---
updated-dependencies:
- dependency-name: tensorflow
  dependency-version: 2.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pyproject.toml

@@ -158,7 +158,7 @@ pin_tensorflow_cpu = [
   "tensorflow; platform_machine=='x86_64' and platform_system == 'Darwin'",
 ]
 pin_tensorflow_gpu = [
-  "tensorflow==2.18.0",
+  "tensorflow==2.21.0",
   # TODO: unpin protobuf after TF is upgraded to 2.21
   # See: https://github.com/tensorflow/tensorflow/pull/103382
   "protobuf<7.34.0",