Release 3.1 requires DUNE > 2.6.0

Congratulations to the 3.1 release. As a release manager of OPM I wanted to test how OPM works together with DUMUX. Somehow I interpreted the website that DUMUX 3.1 should work with DUNE 2.6 also. Unfortunately, that is not the case since (some) of your Cmake code for tests relies on functionality not yet available in stable DUNE releases. With 2.6 on Debian 10 I get the error:

CMake Error at /usr/share/dune/cmake/modules/DuneTestMacros.cmake:201 (message):
  Cannot deduce test name from multiple sources!
Call Stack (most recent call first):
  test/common/geometry/CMakeLists.txt:1 (dune_add_test)


-- Configuring incomplete, errors occurred!

(LABELS for dune_add_test seem to have been introduced after DUNE 2.6.0)

Not sure whether this is a bug feature. If it is the latter, it would be nice to indicate the requirement on DUNE master more clearly, e.g. in https://dumux.org/installation.