build: upgrade to node 20 for the action runtime

Address a deprecation by GitHub:

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: planningcenter/planningcenter/pull-assign-action@v1

References:

https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

action.yml

@@ -6,5 +6,5 @@ inputs:
     required: false
     default: ${{ github.token }}
 runs:
-  using: "node16"
+  using: node20
   main: "dist/index.js"