[fix] update dune version control in constraintsolver to master

Merged Simon Emmert requested to merge fix/version-control-singular-limit-for-master into master

The check of the version control using Dune::FMatrixPrecision::set_singular_limit() was removed only in dune-master and not yet in 2.6, therefore the version control is updated so that the ncpflash and pengrobinson tests pass again.

I hope using 2.7 is the correct way of doing this. It worked for me and all material tests pass with dune-2.6 and dune-master.

Edited by Simon Emmert

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