on the more conventional .yml extension

Is there a source for this? I'm not saying it's wrong but it feels subjective.

I've personally seen it far more frequently than action.yaml in other action repos, e.g. GitHub's own actions/checkout. I didn't bother writing a codesearch, but I would expect occurrences of action.yml to far exceed those of action.yaml across all public GHA repos.

That being said, the main motivation for this change was consistency with 670edba lol