Set action runtime to node20

action.yml was still using node16, which is incompatible with
@actions/core@3, @actions/github@9, and openai@5 (all require Node >=18).
The engines field in package.json already specifies >=20.

Author: johan-j

action.yml

@@ -54,5 +54,5 @@ inputs:
     default: 'https://models.github.ai/inference'
 
 runs:
-  using: 'node16'
+  using: 'node20'
   main: 'dist/index.js'