Implemented.

Not done as a CRON job, instead added to php-info.
A CRON job could be a big pain, if the user chooses to ignore certain checks, they'd get repeated warnings about it. It would also be some work for us to make each check disableable, and track what checks have stopped/started to prevent repeated warnings.
The menu labelling and feature documentation is changed so users know they can do server checks under php-info. Additionally we will do them on the admin dashboard, which the admin is likely to regularly check, especially if some problem has been reported to them.

In addition, workaround for max_input_vars has been added, letting the config module work reliably even if the max_input_vars setting is too low. On other screens it will work too, so long as the form doesn't have an upload field.