Require more recent version of CMake

Dune is compatible with a CMake 3.1, which was released end of 2014. The most current version is 3.17 and it offers tons of features and bug fixes. Since Dune 2.6 we require CMake 3.1. To make it short, it is time to bump the version so something more recent.
What would be the most recent version you felt comfortable as a requirement? Please keep in mind that Dune 2.7 was released three months ago, so probably it will take a couple of months before 2.8 gets released.

I propose one of these versions (please extend):

• 3.10
• 3.12
• 3.13

CMake packages from popular distributions (please extend):

• Debian 10: CMake 3.13
• Ubuntu 18.04 LTS: CMake 3.10.2
• Ubuntu 19.04: CMake 3.13
• Ubuntu 20.04 LTS: CMake 3.16
• RHEL 8: CMake 3.11
• openSuse Leap 15.1: 3.10
• openSuse Leap 15.2 (still beta): 3.13

Important CMake features:

• CMake 3.11: version-files support SameMinorVersion for compatibility checks (important as Dune often increases the minor version, not the major version)
• CMake 3.12: FindPython3 (better Python detecetion)
• CMake 3.13: target_link_libraries, target_compile_options, and other functions for adding target properties work for INTERFACE targets (handy for better Dune build system)