Use Debian 9 as CI job name instead of 9.1

What does this MR do?

While !1947 (merged) started to try to pin Debian 9 to 9.1, which is not forever 😭 , Debian 9 (stretch) is now pointing to 9.11:

https://github.com/debuerreotype/docker-debian-artifacts/tree/74e1a3304401c2eb9c6624ae1056d0a438c15189/stretch

cf. https://hub.docker.com/_/debian

So we can specify Debian as 9 to point to the latest minor version of stretch. Actually we have never pinned it on gitlab-org/gitlab-build-images: https://gitlab.com/gitlab-org/gitlab-build-images/blob/2631b2a1d2f96fd9943d936f54f2b4ce7b27996b/Dockerfile.omnibus-gitlab-stretch#L1

Related issues

n/a

Checklist

See Definition of done.

• [n/a] Changelog entry created. Not applicable for Documentation changes and minor changes.
• [n/a] Documentation created/updated
• [n/a] Tests added
• [n/a] Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com
• [n/a] Equivalent MR/issue for CNG opened if applicable
• trigger-package has a green pipeline running against latest commit

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks