Update the CI ruby version

[alterisian]

Assuming we're not using tzf, what ruby version do we need to define in there ?

Currently fails: #44

3.1.x?

[alterisian]

It looks like the codespace defaults to ruby 3.1.4 - it maybe that this is what we want, or will work for now. But we may need the tzf version dependency. I believe we can setup the codespace (default) to always use the same ruby version.