Set C++ standard to C++20

CMakeLists.txt

src/algebra/TinyMatrix.hpp

src/algebra/TinyVector.hpp

src/language/utils/ASTNodeDataType.cpp

src/mesh/ItemArrayUtils.hpp

src/mesh/ItemValueUtils.hpp

src/mesh/SubItemArrayPerItemUtils.hpp

src/mesh/SubItemValuePerItemUtils.hpp

src/scheme/DiscreteFunctionP0.hpp

src/utils/Array.hpp

tests/test_ASTNodeAffectationExpressionBuilder.cpp

tests/test_ASTNodeFunctionExpressionBuilder.cpp