Remove obsolete CI test systems

The Debian10 container is superseded by the container vae23. Remove the old
container from the test matrix and use onlx the new one.

MR 1599
cherry-picked from b7b6b876

.gitlab-ci.yml

@@ -283,7 +283,6 @@ FileLicenceCheck:
 
 .apptainer_vae23_variables_apr21: &apptainer_vae23_variables_apr21
     CONTAINER: "/cvmfs/vae.gsi.de/vae23/containers/user_container-develop.sif"
-    # CONTAINER: "/cvmfs/vae.gsi.de/vae23/containers/vae23-user_container_20230331T1657.sif"
     OS: vae23
     FAIRSOFT_VERSION: apr21p2
     FAIRROOT_VERSION: v18.6.7