Remove gcc 6 from the list of CI scenarios

It is not supported by Dune 2.8 anymore.

.gitlab-ci.yml

@@ -3,6 +3,13 @@ before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git
 
+.common: &common
+  script: duneci-standard-test
+  artifacts:
+    expire_in: 2 years
+    reports:
+      junit: junit/*.xml
+
 dune:git  clang  C++17:
   image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
   script: duneci-standard-test
@@ -10,7 +17,3 @@ dune:git  clang  C++17:
 dune:git  gcc-8  C++17:
   image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
   script: duneci-standard-test
-
-dune:git  gcc-6  C++14:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-9-gcc-6-14
-  script: duneci-standard-test