Test against Dune 2.8 instead of Dune 2.7

2.7 is simply too old now.  We could even go straight for 2.9,
but let's try how far we get with 2.8.  That one is in current
Ubuntu and Debian releases.

.gitlab-ci.yml

@@ -28,10 +28,10 @@ before_script: &before
              }
     EOT
 
-dune:2.7 gcc:
+dune:2.8 gcc:
   variables:
-    DUNECI_BRANCH: releases/2.7
-  image: registry.dune-project.org/docker/ci/dune:2.7-debian-10-gcc-8-17
+    DUNECI_BRANCH: releases/2.8
+  image: registry.dune-project.org/docker/ci/dune:2.8-debian-10-gcc-8-17
   before_script:
   - *patch-dune-common
   - *before