Update.php now renders with a limited theme registry and will always use the Claro theme

The update.php script has to render things to generate the update UI. This can sometimes lead to an update.php page that will crash.

This occurs when the theme registry loads all modules to gather theme implementations and a module has incorrect data due to updates not completing. (This most commonly occurs when removing custom entity types as an interaction with Views).

In order to remedy this problem, update.php now only loads core's theme implementations (i.e. the 'system' module). To avoid scenarios where people want to theme the update page and might again depend on unstable modules, update.php will now always use the Claro theme.