C++20 support

[white238]

Fixes two compiler errors due to C++20 being more strict than C++17

Incomplete types are not allowed in std::any:

[ 28%] Building CXX object src/gretl/CMakeFiles/gretl.dir/data_store.cpp.o
In file included from /usr/WS2/white238/gretl/repo/src/gretl/data_store.cpp:7:
In file included from /opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/any:39:
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/type_traits:979:21: error: static assertion failed due to requirement 'std::__is_complete_or_unbounded(std::__type_identity<gretl::UpstreamStates>{})': template argument must be a complete class or an unbounded array
  979 |       static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Implicit this is not allowed in lambdas

/usr/WS2/white238/gretl/repo/src/gretl/data_store.cpp:229:54: error: implicit capture of 'this' with a capture default of '=' is deprecated [-Werror,-Wdeprecated-this-capture]
  229 |     std::cout << "eval not implemented for step " << currentStep_ << std::endl;
      |                                                      ^
/usr/WS2/white238/gretl/repo/src/gretl/data_store.cpp:228:24: note: add an explicit capture of 'this' to capture '*this' by reference
  228 |   evals_.emplace_back([=](const UpstreamStates&, DownstreamState&) {
      |                        ^
      |                         , this

[tupek2]

Seems I should be handling error here better. but that is for another day.

[white238]

This PR doesn't change the default C++ standard but does add a CI check to make sure it stays compatible with C++20.