[cmake] Require CMake 3.1 similar to dune-common

enabled an automatic merge when the pipeline for 5425b46e succeeds

merged

mentioned in commit aaaa1414

mentioned in commit 9d3a4052

mentioned in merge request !167 (merged)

Would you please abstain from (automatic) self-merging unless it is for severe and obvious bug fixes.

Sorry, but no, I won't. There was never a decision to not self-merge.

Especially as I fixed the obvious bug of diverging CMake requirements in dune-common and dune-istl.

mentioned in commit 52eb4be3