Move update.php to a more acceptable location

This issue is on the back of #2540416: Move update.php back to a front controller → , so not really relevant until that comes into play, but there are currently still issues.

Currently "update.php" is a route and not an actual file. This causes issues if using a "standard" php nginx config, because it is looking for update.php as a file and not a route. This will be resolved once #2540416: Move update.php back to a front controller → is committed because this moves it to an actual file, however this seems to be adding what is a "core" file, outside core.

It will cause issues for people using drupal/core, for example the template https://github.com/drupal-composer/drupal-project manages a "web" folder where the only php files are index.php and autoload.php, all of the others are in the drupal/core package which gets included by composer. Moving update.php to the root directory gives another file that needs to be managed outside of core and added overhead to those (mainly large enterprise organisations) looking to automate their Drupal 8 builds and deployments with composer.