CI: Update the images used for CI testing

Replace the old ones by what dune-fufem uses currently.

.gitlab-ci.yml

@@ -3,16 +3,12 @@
 before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
 
-dune:git  clang  C++17:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
+dune:git ubuntu-20-04 clang-10  C++17:
+  image: registry.dune-project.org/docker/ci/dune:git-ubuntu-20.04-clang-10-17
   script: duneci-standard-test
 
-dune:git  gcc-9  C++20:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-11-gcc-9-20
-  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 debian-11 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