Bump tensorflow from 2.2.1 to 2.4.0 in /cvat/requirements (#2576)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.2.1 to 2.4.0.
- [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/v2.2.1...v2.4.0)

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cvat/requirements/base.txt

@@ -39,7 +39,7 @@ h5py==2.10.0
 django-cors-headers==3.5.0
 furl==2.1.0
 av==8.0.2 --no-binary=av
-tensorflow==2.2.1 # Optional requirement of Datumaro
+tensorflow==2.4.0 # Optional requirement of Datumaro
 # The package is used by pyunpack as a command line tool to support multiple
 # archives. Don't use as a python module because it has GPL license.
 patool==1.12