Move CI from Singularity to Apptainer

Instead of using the Singularity runtime to execute the test in containers the successor Apptainer is used.

added CI/CD label

requested review from @f.uhlig

assigned to @f.uhlig

requested review from @p.-a.loizeau and removed review request for @f.uhlig

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