Update the documentation of how to create a key

Problem/Motivation

All people that uses this module and that have created a key from within Google Recaptcha Classic got a mail from Google to migrate their keys to Google Cloud. The link in the documentation should be adapted to point to the new doc in Google.
What's more: it isn't free anymore.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes