CI: Add a job that tests with the 2.9 release

2.9 is the latest version available as binary packages.
I like to be able to work with those as much as possible.

.gitlab-ci.yml

@@ -3,6 +3,13 @@
 before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
 
+dune:2.9 debian-11 gcc-10 C++20:
+  variables:
+    DUNECI_BRANCH: releases/2.9
+  image: registry.dune-project.org/docker/ci/dune:2.9-debian-11-gcc-10-20
+  before_script:
+  script: duneci-standard-test
+
 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