Move CI to GitHub Actions

[tombruijn]

Update RuboCop to a more recent version

Which runs on more recent Ruby versions too.

Move CI to GitHub Actions

In the process I've removed old Rails 4.0 versions because they're long EOL and I can't make it work locally or on the CI.

It doesn't test on all the EOL Ruby versions, but only tests on the Ruby 2.6 or 2.7 depending on the Rails version it tests agains.

I've not migrated danger as there's another PR open for that.

Set up a local test environment

Add scripts to test the test suite on different Ruby version without having them all installed locally.

Fix tests for new Ruby version

Something changed since the Ruby version we're now testing with, so update the specs to match.

[dblock]

Looks like we're losing Rails 5 too, but YOLO? This looks like it's passing CI in https://github.com/mongoid/mongo_session_store/actions/runs/21883800394 - merge?

For Danger, we have #48.

[tombruijn]

@dblock Let me tinker a bit more before merge. I first tried to get a minimal passing build.

I also had a look at adding Rails 8.x but that's going to require some library/spec updates from what I can tell so far.

[dblock]

Makes sense, would be good to get exactly where CI was before this change, not remove/add anything.

[tombruijn]

@dblock I've updated the PR to include Rails 5.x for now too. Rails 4 I can't make work locally or on the CI and I don't want to spend more time on it. So let's drop support.

Newer versions of Rails will need some more work. I'll create a separate PR for that.