[ci] Update the ci to use dune 2.9

.gitlab-ci.yml

@@ -5,7 +5,7 @@ stages:
 variables:
   IMAGE_REGISTRY_URL: $CI_REGISTRY/dumux-repositories/dumux-docker-ci
   DUMUX_CI_TRIGGER_LECTURE_BRANCH: master
-  DUMUX_CI_DUNE_LATEST_RELEASE: "2.8"
+  DUMUX_CI_DUNE_LATEST_RELEASE: "2.9"
 
 # Cases in which to create pipelines at all. The trigger jobs may further
 # specify how exactly they should be created in different situations.