PHP 8.4: Drupal8Post::submit() missing return type declaration causes fatal error

Problem

Running Drupal 10.6.x with PHP 8.4 causes a fatal error on page load:

Fatal error: Declaration of Drupal\recaptcha\ReCaptcha\RequestMethod\Drupal8Post::submit(
  ReCaptcha\RequestParameters $params
) must be compatible with ReCaptcha\RequestMethod::submit(
  ReCaptcha\RequestParameters $params
): string in Drupal8Post.php on line 39

PHP 8.4 enforces return type declarations on methods that implement
an interface. The RequestMethod interface declares submit() with
a : string return type, but Drupal8Post::submit() omits it.

Steps to reproduce

1. Install Drupal 10.6.x
2. Enable PHP 8.4
3. Install drupal/recaptcha 8.x-3.x
4. Visit any page — fatal error is thrown immediately

Environment

• Drupal: 10.6.x
• PHP: 8.4.x
• Module: drupal/recaptcha 8.x-3.4
• Server: nginx / platform.sh

Proposed fix

In src/ReCaptcha/RequestMethod/Drupal8Post.php line ~39:

// Before
public function submit(RequestParameters $params)

// After
public function submit(RequestParameters $params): string

A merge request with this fix is attached.