Add basic Salsa-CI for project quality assurance

Read more at https://salsa.debian.org/salsa-ci-team/pipeline#basic-use

You can see this in action at https://salsa.debian.org/otto/atop/-/pipelines/272817

To enable Salsa-CI, you need the following Gitlab settings changed:

1. Enable CI in https://salsa.debian.org/debian/atop/edit > Visibility, project features, permissions
2. Define Gitlab-CI file name in https://salsa.debian.org/debian/atop/-/settings/ci_cd > General pipelines as debian/salsa-ci.yml