Use 1 release per engine-version to avoid the 1000 assets per release limit

[eregon]

See #752 (comment)

I'll take this occasion to cleanup the platform names and mostly match https://github.com/actions/python-versions/releases/tag/3.13.6-16792117939 which has a similar design.
No plan to have a versions-manifest.json though, that seems too inefficient and easy to get out of sync.

[eregon]

Running the copy to separate releases in https://github.com/ruby/ruby-builder/actions/runs/17892621119/job/50875027881

[eregon]

Done with https://github.com/ruby/setup-ruby/releases/tag/v1.262.0.
In *-dev-builder repo we can stop copying to old platforms in some time.

[eregon]

We should also follow https://github.com/ruby/ruby-builder?tab=readme-ov-file#process-for-new-builds probably.