Bump tensorflow from 2.8.0 to 2.9.3 in /examples/mnist-keras (#458)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.8.0 to 2.9.3.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.8.0...v2.9.3)

---
updated-dependencies:
- dependency-name: tensorflow
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/mnist-keras/requirements.txt

@@ -1,3 +1,3 @@
-tensorflow==2.8.0
+tensorflow==2.9.3
 fire==0.3.1
 docker==5.0.2