Update Node to 24

* There are warnings for Node 20 now, see
  #806 (comment)
* Node 20 is deprecated now and EOL in April 2026.
  Node 24 has been available for a long time, so no runners should lack node24.
  See https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/
* For very old runners without node24 it's still possible to
  force using node20 with `ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION=20`.
  See also the blog post above.

Author: eregon

action.yml

@@ -55,5 +55,5 @@ outputs:
   ruby-prefix:
     description: 'The prefix of the installed ruby'
 runs:
-  using: 'node20'
+  using: 'node24'
   main: 'dist/index.js'