Upgrade Node to v20.

For context: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

Fixes hpi-swa/smalltalkCI#627

Author: fniephaus

action.yml

@@ -22,7 +22,7 @@ outputs:
   smalltalk-version:
     description: 'Deprecated, please use smalltalk-image'
 runs:
-  using: 'node16'
+  using: 'node20'
   main: 'dist/index.js'
 branding:
   icon: 'box'