Run CI checks against Ruby 3.1

[eoinkelly]

Fixes #445. Includes changes from #441 (which are required to successfully build now) so will wait for #441 to land before making this ready for merge.

[G-Rath]

I think we should continue to run against the latest patch version of Ruby (rather than locking via .ruby-version) because in theory it should always work, and that should help catch if it doesn't (remember we also run CI once a week which notifies at least me if it fails).

Ideally sort of given #448 we should probably run against at least a couple of versions of Ruby to ensure compatibility - ideally ideally it would be good to run against say most recent stable and upcoming latest, though when @robotdana set that up on another public project of ours it seemed very unstable but maybe there's some way we could pull in like "latest prerelease" or something rather than the most latest commit version of Ruby?

[eoinkelly]

@G-Rath I have a slight preference for a .ruby-version file because it automatically syncs up local dev and CI but I also see the value of your approach. I'm fine with making the smaller change (your approach). It looks like #444 is unblocked now so I'll update this PR to run CI against 3.2 instead of 3.1

[eoinkelly]

Closing in favour of #452