Gitlab-CI: Adopt stage list to upstream Salsa-CI changes

This has no functional changes to the Gitlab-CI test run in MariaDB, this
is purely to satisfy the gitlab-ci.yml linter.

debian/gitlab-ci.yml

@@ -17,7 +17,8 @@ stages:
   - upgrade in Stretch
   - upgrade from Jessie
   - test extras
-  - test # Stage referenced by reprotest stanza, so must exist
+  - test # Stage referenced by Salsa-CI template reprotest stanza, so must exist
+  - publish # Stage referenced by Salsa-CI template aptly stanza, so must exist even though not used
 
 build:
   extends: .build-package