[ci] Drop no longer supported CI configurations

These configurations are outdated for two reasons:

* Clang versions before 10 are no longer supported.
* Ubuntu 18.04 has reached its EOL in 2023-06.

.gitlab-ci.yml

@@ -31,25 +31,6 @@ debian-11-gcc-9-17-with-checking:
     DUNECI_CMAKE_FLAGS: "-DDUNE_MAX_TEST_CORES=4 -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
   tags: [duneci]
 
-# no numpy version available for this image on the gitlab registry
-# disable python bindings for now
-ubuntu:18.04 clang-6-17:
-  image: registry.dune-project.org/docker/ci/ubuntu:18.04
-  script: duneci-standard-test
-  stage: test
-  variables:
-    DUNECI_TOOLCHAIN: clang-6-17
-    DUNECI_CMAKE_FLAGS: "-DDUNE_ENABLE_PYTHONBINDINGS=OFF"
-  tags: [duneci]
-ubuntu:18.04 clang-5-17:
-  image: registry.dune-project.org/docker/ci/ubuntu:18.04
-  script: duneci-standard-test
-  stage: test
-  variables:
-    DUNECI_TOOLCHAIN: clang-5-17
-    DUNECI_CMAKE_FLAGS: "-DDUNE_ENABLE_PYTHONBINDINGS=OFF"
-  tags: [duneci]
-
 # Check for spelling mistakes in text
 code-spelling-check:
   stage: .pre