[bugfix][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.

In !249 (merged) it became obvious, that the 2.10 jobs don't use 2.10. This should be fixed with the present MR. If there is interest in testing against 2.10 with clang, we first need an image a corresponding image.

enabled an automatic merge when all merge checks for 2aa99ae6 pass

mentioned in merge request !249 (merged)

merged

mentioned in commit f598f033