Include GCC v5 and GCC v6 in the CI

[dimztimz]

Description GitHub actions provides two Linux images, one is Ubuntu 18.04 which provides GCC 5-8 and 20.04 which has GCC 7-10. One can tweak the scripts to use the older Ubuntu for the older GCC versions. Or we can always use the older Ubuntu and get the newer GCC versions via PPA.