Update inherited image version

[saffaalvi]

The version of the image we currently inherit from jupyter/docker-stacks in base-notebook is outdated.
Good idea to update this to the latest version before proceeding with pinning any packages.

[saffaalvi]

The latest image build is jupyter/datascience-notebook:6d42503c684f which uses Python 3.8.5.
When trying to update base-notebook-cpu to this image, there was an error with kubeflow-metadata and ml-metadata:

ERROR: Could not find a version that satisfies the requirement ml-metadata==0.21.1 (from kubeflow-metadata==0.3.1) (from versions: 0.12.0.dev0, 0.13.0.dev0, 0.13.1.dev0)
ERROR: No matching distribution found for ml-metadata==0.21.1 (from kubeflow-metadata==0.3.1)

This issue has been brought up already google/ml-metadata#37

There is only one version after the one we are currently using that uses Python 3.7.6 (https://github.com/jupyter/docker-stacks/wiki/datascience-notebook-54462805efcb) but there doesn't seem to be any significant changes between the two versions.