Allow opt-in PHP 8.5 testing

There's following settings https://git.drupalcode.org/project/gitlab_templates/-/blob/main/includes...

  # The minimum supported version of PHP for the current stable version of Drupal.
  CORE_PHP_MIN: '8.3'

  # The maximum/latest supported version of PHP for the current stable version of Drupal.
  CORE_PHP_MAX: '8.4'

  # The next/prerelease version of PHP targeted for support by the current stable version of Drupal.
  CORE_PHP_NEXT: '8.4'

  # The minimum version of PHP targeted for support by the next Major release of Drupal.
  CORE_NEXT_PHP_MIN: '9.0'

  # The maximum/latest version of PHP targeted for support.
  CORE_NEXT_PHP_MAX: '9.0'

  # The next/prelease version of PHP targeted for support by the next Major release of Drupal.
  CORE_NEXT_PHP_NEXT: '9.0'

So after 11.2 the `CORE_PHP_NEXT` should be increased to 8.5

Wait till 11.2 release