[!501] Feature/cleanup 2.6 deprecations
Merge branch 'feature/cleanup-2.6-deprecations' into 'master'
ref:pdelab/dune-pdelab> We remove support for deprecated features.
This should be backported to 2.7
See merge request pdelab/dune-pdelab!501
(cherry picked from commit f5eaa4e3)
80f31f01 [cleanup] remove compatibility code for dune 2.3 52d9557e [doc] update FEM interface to show the best-practice approach with stati ocnstexpr functions. 261d0f33 [FEM] remove compatibility code for PDELab < 2.6 cabae42b [cleanup] remove deprecated PowerL2 operator