Filters whether to attempt to perform the multisite DB upgrade routine.
In single site, the user would be redirected to wp-admin/upgrade.php. In multisite, the DB upgrade routine is automatically fired, but only when this filter returns true.
If the network is 50 sites or less, it will run every time. Otherwise, it will throttle itself to reduce load.
(bool) Whether to perform the Multisite upgrade routine. Default true.