cron_execute_plugin_type should report that plugins are using "old" cron implemenation

It was proposed as part of MDL-60683 that the quiz module & it's sub-plugins should move to scheduled tasks, and it would be desirable that a notice was raised if any of its sub-plugins were still using the cron.

This is a proposal for a general fix, instead of replicating much of this code to simply output a DEBUG_DEVELOPER message.

The cron_execute_plugin_type() }}function in {{/lib/cronlib.php is used to find out

1. if a particular plugin implements the *_cron() function in its lib.php
2. Executes the identified *_cron() functions

This would also produce as part of the legacy_cron_plugin_cron_task task a list of plugins using the legacy cron system