COMP: Upgrade Azure CI from ubuntu-16.04 to ubuntu-18.04
COMP: Upgrade Azure CI from ubuntu-16.04 to ubuntu-18.04

Addressed:

> ##[warning]Ubuntu 16.04 LTS environment is deprecated and will be removed on September 20, 2021. Migrate to ubuntu-latest instead. For more details, see https://github.com/actions/virtual-environments/issues/3287.

From https://dev.azure.com/kaspermarstal/Elastix/_build/results?buildId=2229&view=results

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix