Bump tensorflow from 2.1.2 to 2.5.1 in /elasticdl_preprocessing (#2523)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.1.2 to 2.5.1.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.1.2...v2.5.1)

---
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>

Author: dependabot[bot]

elasticdl_preprocessing/requirements.txt

@@ -1 +1 @@
-tensorflow==2.1.2
+tensorflow==2.5.1