Bump tensorflow from 2.4.1 to 2.5.0 in /backend

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.4.1 to 2.5.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.4.1...v2.5.0)

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Author: dependabot[bot]

backend/requirements.txt

@@ -51,7 +51,7 @@ six==1.15.0
 structlog==21.1.0
 tensorboard==2.4.1
 tensorboard-plugin-wit==1.8.0
-tensorflow==2.4.1
+tensorflow==2.5.0
 tensorflow-estimator==2.4.0
 termcolor==1.1.0
 tinycss2==1.1.0