Deprecation warning about WebformTwigExtension

Problem/Motivation

Running a test of an unrelated custom module on D11.4/PHP 8.3 results in this warning:

Method "Twig\Extension\ExtensionInterface::getFunctions()" might add "array" as a native return type declaration in the future. Do the same in implementation "Drupal\webform\Twig\WebformTwigExtension" now to avoid errors or add an explicit @return annotation to suppress this message.

Steps to reproduce

Test a custom or contrib module on D11.4/PHP 8.3

Proposed resolution

The interface in Twig 3.28.0 doesn't have a return value. You could add it there, add it to the webform method, add '@return', add 'ReturnTypeWillChange'...

Since this is such a small change it's probably best to incorporate into something else.