Bump tensorflow from 1.2.1 to 2.3.1 in /keras-mnist/mnist_cnn
Bump tensorflow from 1.2.1 to 2.3.1 in /keras-mnist/mnist_cnn

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.2.1 to 2.3.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](https://github.com/tensorflow/tensorflow/compare/v1.2.1...v2.3.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
yoshiweb/keras-mnist