Feature/adjust c++ checks

added 6 commits

• 3818fcb8 - [cmake] Don't check for C++17 features which we require anyway
• 6d4fc7fc - Deprecate utility.hh
• ea1f3980 - Remove deprecated memory.hh
• a87fa6da - Deprecate variant.hh
• 196b8e39 - Deprecate optional.hh
• 5585fed6 - [alignedallocator] Remove use of posix_memalign

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I don't know why the CI fails for GCC 7: Both the code and the CMake check for the C++ version. I tested with my local GCC 7.5 and both parts are working. Not sure whether it is the libstdc++ version that is newer on my system…

added 13 commits

• 5585fed6...57b4aee0 - 5 commits from branch master
• 7291cc62 - [cleanup] Remove old defines used until Dune 2.4
• c6ba736d - [cmake] Adjust test to require C++17
• a7de766c - [cmake] Don't check for C++17 features which we require anyway
• bbc620a3 - Deprecate utility.hh
• ff57b466 - Remove deprecated memory.hh
• de9b9b36 - Deprecate variant.hh
• 8b684f96 - Deprecate optional.hh
• 40ed63fd - [alignedallocator] Remove use of posix_memalign

Compare with previous version

merged

mentioned in commit 27059e0f

mentioned in merge request !786 (merged)