Update CI to test against dune 2.8 after rasing requirement

.gitlab-ci.yml

@@ -13,11 +13,11 @@ before_script:
   - duneci-install-module https://gitlab.dune-project.org/core/dune-geometry.git
   - duneci-install-module https://gitlab.dune-project.org/core/dune-grid.git
 
-dune:2.6 ubuntu:18.04 clang-6-17:
-  image: registry.dune-project.org/docker/ci/ubuntu:18.04
+dune:2.8 ubuntu:20.04 clang-10-20:
+  image: registry.dune-project.org/docker/ci/ubuntu:20.04
   script: duneci-standard-test
   variables:
-    DUNECI_TOOLCHAIN: clang-6-17
+    DUNECI_TOOLCHAIN: clang-10-20
     DUNECI_CMAKE_FLAGS: "-DDUNE_PYTHON_VIRTUALENV_SETUP=1"
-    DUNECI_BRANCH: releases/2.6
+    DUNECI_BRANCH: releases/2.8
   tags: [duneci]