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As Ubuntu-1604 runners will no longer be available in early May 2022
Update `ubuntu-1604:201903-01` to `ubuntu-2004:202104-01`
and `ubuntu-1604-cuda-10.2:202012-01` to `ubuntu-2004-cuda-11.4:202110-01`
Per [CircleCI Configuration reference](https://circleci.com/docs/2.0/configuration-reference/)