feat: upgrade from node16 to node20 (#8)

Node 16 has reached end of life, so GitHub is deprecating it and adding
warnings to any actions that use it.

action.yml

@@ -12,6 +12,6 @@ inputs:
  required: false
 
 runs:
- using: node16
+ using: node20
  main: dist/main/index.js
  post: dist/post/index.js