[RLlib](deps): Bump tensorflow in /python/requirements/rllib

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.4.1 to 2.5.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.4.1...v2.5.0)

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

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

Author: dependabot[bot]

python/requirements/rllib/requirements_rllib.txt

@@ -1,6 +1,6 @@
 # Deep learning.
 # --------------
-tensorflow==2.4.1
+tensorflow==2.5.0
 tensorflow-probability==0.12.2
 torch==1.8.1;sys_platform=="darwin"
 torchvision==0.9.1;sys_platform=="darwin"