Bump to node20 (#33)

node16 is deprecated, see https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

action.yml

@@ -16,5 +16,5 @@ outputs:
   content:
     description: 'File content'
 runs:
-  using: 'node16'
+  using: 'node20'
   main: 'dist/index.js'