do not test the builds on ubuntu 16:04 and debian stretch

[Remi-Gau]

this may be kinder to our CI as it would dramatically reduce the size of the multiverse we are currently "exploring"

the question is also: do we want to support those "old" base image?

https://github.com/ReproNim/neurodocker/blob/9e235e0c55e797d8a1585fbe911a6e9de6766668/.github/workflows/create_workflows.py#L31-L38

[stebo85]

agree, that limiting the tests to the latest base images would make sense: reducing the load on testing and encouraging security best practices.