Update default runtime to node16

Node 12 has an end of life on April 30, 2022.

This PR updates the default runtime to [node16](https://github.blog/changelog/2021-12-10-github-actions-github-hosted-runners-now-run-node-js-16-by-default/), rather then node12. 

This is supported on all Actions Runners v2.285.0 or later.

Author: thboop

action.yml

@@ -27,5 +27,5 @@ outputs:
   result:
     description: The return value of the script, stringified with `JSON.stringify`
 runs:
-  using: node12
+  using: node16
   main: dist/index.js