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/filesystem.hh

@@ -6,14 +6,16 @@
 #if HAVE_GHC_FILESYSTEM
 
 #include <ghc/filesystem.hpp>
-
+namespace Dune::Copasi {
 namespace fs = ghc::filesystem;
-
+}
 #else
 
 #include <filesystem>
 
+namespace Dune::Copasi {
 namespace fs = std::filesystem;
+}
 
 #endif