mod_scheduler: Fix Scheduler to use reCAPTCHA v2

When booking in a slot (with recaptcha enabled), the submitted form data still uses  recaptcha_challenge_field, which is V1 for reCAPTCHA. This means that even if the user selects the validation/image challenge correctly, the message "Failed" will always be prompted.

This updates the mod_scheduler validation function for reCAPTCHA to use V2 implementation using g-recaptcha-response.