Improve CI to test against multiple versions of Rails

[okuramasafumi]

Here are multiple improvements on CI. The most important part is adding support for multiple Rails versions, but also:

• Using ubuntu only
  • Test matrix is already large, and there is little benefit to test against windows and mac
• Do not test against macos-13 since it doesn't work now
• Use checkout@6

We cannot merge this now. Once we agree this approach, we need to remove the first commit on this branch since it will be redundant.

[makicamel]

Thank you!