Migrate to GitHub Actions CI, fix test builds on main

[grega]

Two issues being resolved here:

• Test builds have started failing on main (due to Gems being installed that are incompatible with Ruby 2.7.7), see note from Patrick below on resolution
• Migrating away from Circle CI and on to GitHub Actions (now our preferred CI platform)

[github-actions]

• GitHub Actions run #7555431855

Test coverage: 100%

[patch0]

@grega @ce-rpf I've got this testing now with ruby 2.7, 3.1, 3.2, 3.3 and rails 6.1, 7.0, 7.1.

There is a fly in the ointment, which is a lack of default Gemfile. I've updated the README to say that BUNDLE_GEMFILE needs to be exported as an env var, but this is quite a trip hazard compared to regular gems. Using symlinks doesn't work so well, as remote .. in the gemfiles/*.gemfile.lock gets changed to remote . if the Gemfile is symlinked as it was before, leading to test failures (as the Gemfile.lock gets changed).

The other thing is that I've set the .tool-versions ruby version to 2.7.8, which is the lowest version we support. Some of the gems get updated to versions that are not compatible with 2.7 if this is left at 3.x. I guess the next step would be more gemfiles to cover all the combinations of rails and ruby we need coverage for, but it seems fine at the mo with this version set.

[grega]

Note to re-enable Rubocop check is here: #67