upgrade to node 16 (game-ci#193)

node12 actions are deprecated.
See documentation here:
https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

Currently, the unity builder action gives me the following warning:
```
Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: game-ci/unity-test-runner, game-ci/unity-builder
```

Author: paulpach

action.yml

@@ -61,5 +61,5 @@ branding:
   icon: 'box'
   color: 'gray-dark'
 runs:
-  using: 'node12'
+  using: 'node16'
   main: 'dist/index.js'