[tune](deps): Bump tensorflow-estimator in /python/requirements/ml

Bumps [tensorflow-estimator](https://github.com/tensorflow/estimator) from 2.6.0 to 2.8.0.
- [Release notes](https://github.com/tensorflow/estimator/releases)
- [Commits](tensorflow/estimator@v2.6.0...v2.8.0)

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

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

python/requirements/ml/requirements_rllib.txt

@@ -20,7 +20,7 @@ supersuit==3.3.2; python_version >= '3.7'
 pybullet==3.2.0
 # For tests on RecSim and Kaggle envs.
 recsim==0.2.4
-tensorflow_estimator==2.6.0
+tensorflow_estimator==2.8.0
 # DeepMind's OpenSpiel
 open-spiel==1.0.2