.gitlab-ci.yml: test with GCC 9 with C++20

.gitlab-ci.yml

@@ -7,6 +7,10 @@ dune:git  clang  C++17:
   image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-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
   script: duneci-standard-test