Introduce working CI/CD through GitHub Actions

[iBotPeaches]

Problem

CI/CD has decayed for 2 reasons. The images are no longer supported (ubuntu-20) and the actions used (checkout-v1) are no longer supported. This means they instant fail.

Solution

1. We introduce configuration with Dependabot to manage GitHub Actions which means each time an action releases a new major release (1-2 a year) - a PR will be delivered.
2. We rework the pipelines entirely. In 2026 setup-ruby is far more powerful with caching built in and more. We leverage that in order to reduce the code needed for manual gem cache creation. This turns CI pipelines into about 6 lines of code.
3. We expand testing from minimal Ruby version to as high as we can go (3.3). 3.4/4.0 require changes due to the standard gem removal.
4. We expand the Mac testing on both Mac 14 and 26 in order to get a CI run with the newer restrictive Xcode 26.

Meta

Until my PR is approved for CI to run - you'll have no proof it works. So here is my fork and a test PR for proof. You can see failures for Ruby 3.4 & 4.0 here which I can do a follow-up PR for post merge.