Switch action's code to run on Node 16

GH is deprecating usage of `node12` in custom GH Actions. Using an action which runs on `node12` throws a warning:
```
The following actions uses node12 which is deprecated and will be forced to run on node16: tretuna/sync-branches@1.4.0. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/
```