Makes KAMG and FastAMG solver available with dune-istl 2.3

Previously we relied on the define DUNE_HAS_FAST_AMG to detect
whether these preconditioners are available. This define is only
available in the 2.2 release with cmake support. Therfore we now
addtionally test whether we are using dune-istl 2.3 or newer.

With this PR the new solvers become available for dune-istl 2.3 and above!