GHA: Need for test with different c++ versions

[robbietuk]

This came up in a discussion today so making a reminder issue. I think we are only testing c++11 builds with GHA. https://github.com/UCL/STIR/blob/master/.github/workflows/build-test.yml

We found that with some MacOS clang versions, with c++14/c++17/c++20, were producing build warnings or errors in https://github.com/UCL/STIR/issues/856, which was fixed in https://github.com/UCL/STIR/pull/987