[ci][tests] Increase default build/test concurrency to 8 tasks

5 jobs for !2790 with feature/ci-8-parallel-tasks
latest detached
Status Name Job ID Coverage
  Trigger
manual full-dune-2.7-clang
manual full-dune-2.7-gcc
manual full-dune-master-gcc
manual minimal-dune-2.7-gcc
 
  Downstream Modules
created trigger lecture