Update Psychec's CI to use C++17

[aytey]

Is your feature request related to a problem? Please describe.

PR #40 "fell over" because it used the legally-allowed C++17 header filesystem, but this does not actually exist in psychec CI base image:

• https://askubuntu.com/questions/1256440/how-to-get-libstdc-with-c17-filesystem-headers-on-ubuntu-18-bionic

Describe the solution you'd like

Update the GitHub actions to use a more-modern Ubuntu or look at using a later GCC within 18.04

Describe alternatives you've considered

Change PR #40 to not use filesystem (even though it is in C++17 😞)

Additional context

Build log from #40: https://github.com/ltcmelo/psychec/pull/40/checks?check_run_id=2161399189#step:4:65

[ltcmelo]

I didn't know about such ubuntu "situation"… it appears that it needs <experimental/filesystem> (with extra link flags). But I think this can be handled in the Cmake file and with a preprocessing directive to choose between <filesystem> and <experimental/filesystem> ?

(Added further comments in the PR)