Add new tests on macosx 10.15 and 11

Add tests for macosx 10.15 and 11 on our new runners. Retire old test on macosx 10.13.

Define number of CPUs in runner configuration.

Move the definition from the global gitlab CI configuration to the local runner configurations. Defining the number of CPUs in the runner configuration allows to adjust this value to the actual underlying hardware.

added CI/CD label

mentioned in merge request !397 (closed)

This MR combines all commits of !397 (closed) into one. !397 (closed) could not be merged any longer probably due to some forced push.

Dear @f.uhlig, @v.friese, @p.-a.loizeau,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

approved this merge request

merged