Restrict samples testing to only latest Ruby version

[opoudjis]

The time to test samples for metanorma-cli release is only going to get longer, especially with monster samples repositories like BIPM, BSI, and Plateau involved. I propose that we only test releases on the latest version of Ruby for samples repositories from now on. @ronaldtse agrees.

We should also make v3.4 compilation mandatory for Ruby, now that compilation appears to be passing on it in GHA...