Bump keras in /openfl-workspace/keras/tensorflow/mnist (#1444)

Bumps [keras](https://github.com/keras-team/keras) from 3.8.0 to 3.9.0.
- [Release notes](https://github.com/keras-team/keras/releases)
- [Commits](keras-team/keras@v3.8.0...v3.9.0)

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

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

Author: dependabot[bot]

openfl-workspace/keras/tensorflow/mnist/requirements.txt

@@ -1,2 +1,2 @@
-keras==3.8.0
+keras==3.9.0
 tensorflow==2.18.0