Modernize the CI setup

.gitlab-ci.yml

@@ -18,12 +18,12 @@ dune:2.9  clang  C++20:
   variables:
     DUNECI_BRANCH: releases/2.9
 
-dune:git  clang  C++17:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
+dune:git  clang-11  C++20:
+  image: registry.dune-project.org/docker/ci/dune:git-debian-11-clang-11-20
   script: duneci-standard-test
 
-dune:git  gcc-8  C++17:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
+dune:git  gcc-10  C++20:
+  image: registry.dune-project.org/docker/ci/dune:git-debian-11-gcc-10-20
   script: duneci-standard-test
 
 # Check for spelling mistakes in text