Update CI config to clang-7

The old image is no longer updatet
3 jobs for feature/update-to-clang-7 in 9 minutes (queued for 47 seconds)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #29085

00:05:48

passed dune:git gcc-6 C++14 #29087

00:03:38

passed dune:git gcc-8 C++17 #29086

00:04:46