upgrade to Node 20

This corrects:

```

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: ./. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
```

Signed-off-by: Mike Ball <mikedball@gmail.com>

action.yml

@@ -9,5 +9,5 @@ inputs:
     required: true
     description: 'A GitHub access token'
 runs:
-  using: 'node16'
+  using: 'node20'
   main: 'dist/index.js'