CI: upgrade vers ci-config 6.0.0.

.gitlab-ci.yml

 include:
   # cf. https://gitlab.irstea.fr/pole-is/tools/ci-config
   - project: pole-is/tools/ci-config
-    ref: "4.0.2"
-    file: /php/multiversions.yml
-  - project: pole-is/tools/ci-config
-    ref: "4.0.2"
-    file: /php/packagist.yml
+    ref: "6.0.2"
+    file:
+      - /php/multiversions.yml
+      - /php/packagist.yml