Consider testing against multiple Rails versions

[andyw8]

Currently we run CI against the latest release release, and against main, but I'm considering if we should also test against older supported Rails versions, such as 7.2, since there are a number of places where we rely on feature discovery using responds_to?.

This would probably mean having multiple dummy apps in the repo, e.g. test/dummy_rails_8 and test/dummy_rails_7_2.