Switch back to node 12

Node 16 is only available on GHES 3.4+, which was released relatively
recently. This Action still works with Node 12, so switching back to
that version widens compatibility for self-hosted runners users.

action.yml

@@ -11,5 +11,5 @@ inputs:
     description: '(optional) The ID of a pull request to auto-approve. By default, this action tries to use the pull_request event payload.'
     required: false
 runs:
-  using: 'node16'
+  using: 'node12'
   main: 'dist/index.js'