I haven’t spent the time to scope the change, but I did note here where most of the changes would live:
If this upgrade also changes the way ReCAPTCHA is exposed in the UI, this is currently scattered across multiple Form implementations. For example:
This would certainly be welcome as a pull request if you are up for it. Remember to ensure backwards compatibility by making the new version optional, such as adding a new Config variable for which version of ReCAPTCHA is configured in config.inc.php.
Feel free to reach out with any specific questions.