Bump keras from 2.3.1 to 2.4.2 in /cvat/requirements (#1760)

Bumps [keras](https://github.com/keras-team/keras) from 2.3.1 to 2.4.2.
- [Release notes](https://github.com/keras-team/keras/releases)
- [Commits](https://github.com/keras-team/keras/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

cvat/requirements/base.txt

@@ -39,7 +39,7 @@ cython==0.29.20
 matplotlib==3.0.3
 scikit-image==0.15.0
 tensorflow==1.15.2
-keras==2.3.1
+keras==2.4.2
 opencv-python==4.1.0.25
 h5py==2.9.0
 imgaug==0.4.0