Support libc++ and reduce minimum required standard to c++20

• Use alternative to std::move_only_function from function2 library
  • head only library
  • provides unique_function
• Issues with using std::move_only_function:
  • requires c++23 with libstdc++
  • not yet implemented by libc++
• Reduce minimum required standard in CMakeLists.txt to c++20
• Add missing lambda parentheses

dune/copasi/common/tiff_grayscale.hh

@@ -3,6 +3,8 @@
 
 #include <dune/copasi/common/tiff_file.hh>
 
+#include <function2/function2.hpp>
+
 #include <functional>
 #include <queue>
 
@@ -60,7 +62,7 @@ class TIFFGrayscale
     TIFFFile const* _tiff_ptr;
     std::size_t _row;
     std::size_t _max;
-    std::move_only_function<std::size_t(std::size_t) const DUNE_COPASI_FUNCTOR_NOEXCEPT>
+    fu2::unique_function<std::size_t(std::size_t) const noexcept>
       _read_col{};
   };