with-post-step: use Node.js 20 instead of Node.js 16

(cherry picked from commit 0c1e72cfd6)

with-post-step/action.yml

@@ -37,6 +37,6 @@ inputs:
     default: POST
 
 runs:
-  using: 'node16'
+  using: 'node20'
   main: 'main.js'
   post: 'main.js'