[CMake] add version defines to the config.h.cmake

When checking for a certain dune-typetree version using
DUNE_VERSION_NEWER, the defines where not present and
the makro did not work.

Kudos to Andreas Nüßing for noticing the problem and providing a patch
for PDELab.
Documentation] Update release notes and README
Merge branch 'releases/2.3'

Pull in fix

* releases/2.3:
  [CMake] add version defines to the config.h.cmake