[CI] Use 2.10 image for the jobs testing against 2.10

Unfortunately the clang images for 2.10 use a clang version
that we no longer support.

.gitlab-ci.yml

@@ -15,16 +15,12 @@ dune:2.9 debian-11 gcc-10 C++20:
   script: duneci-standard-test
 
 # The 2.10 release is the one in Debian 13 ('trixie', 'testing' at the time of writing)
-dune:2.10 debian-11 clang-13  C++20:
-  variables:
-    DUNECI_BRANCH: releases/2.10
-  image: registry.dune-project.org/docker/ci/dune:git-debian-11-clang-13-20
-  script: duneci-standard-test
-
+# To test against 2.10 we need an image with the corresponding version of the core modules.
+# Unfortunately, there is no 2.10 image with a recent enough compiler.
 dune:2.10 debian-11 gcc-10  C++20:
   variables:
     DUNECI_BRANCH: releases/2.10
-  image: registry.dune-project.org/docker/ci/dune:git-debian-11-gcc-10-20
+  image: registry.dune-project.org/docker/ci/dune:2.10-debian-11-gcc-10-20
   script: duneci-standard-test
 
 # Also test with the current development branch