Update firebase dependency to latest major version

This MR updates the external firebase/php-jwt dependency to the latest available major release.

As of version 7, the firebase/php-jwt package enforces a minimum length for the secret used to encode JWT. This might break sites using the module and have set too short secrets.

Therefore, a hook_requirements is added to warn users about the new requirement in the Drupal status report. It should also be mentioned in the release notes and potentially we need to create new major release.