Add Ruby 3 to Circle tests

[jordan-brough]

Start testing the code on Ruby 3, without making it the default yet.

If we get to a point where there are gem conflicts for ruby5x that won't even install under Ruby 3 then we'd need to do something more complex, but this should work for now I think, and get Ruby 3 under test coverage at least?

Notes:

• We have to exclude Rails < 6.0 in .circleci/config.yml when running tests on Ruby 3, since only Rails 6 and above support Ruby 3.
• ✅ This won't actually run any of the rspec tests on Ruby 3 until Fix rspec invocation in CircleCI #2125 is merged, since the tests aren't currently actually running on Circle for Rails >= 6 and Ruby 3 only runs on Rails >= 6.

[nickcharlton]

Thanks for this and all of the others I've merged in in the last few hours, it's really helpful and I appreciate how you've single-purposed each of your PRs, along with the detailed descriptions. It's made it so much easier to jump in and catch up with things!

I've just adjusted this to use the new style Circle blocks which is something I merged in from #2102.

[jordan-brough]

Sounds great! Thanks for the quick responses!