Remove any non legacy cron from Moodle standard distribution

Since it is deprecated as of MDL-52846

The orginal goal was to do this for the 3.5 release, but in the end, some of the subtasks below were not done until Moodle 3.7.

Some (not all) areas that use it are -
++ Use of legacy cron is deprecated (repository_dropbox_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (repository_filesystem_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (\block_rss_client::cron()). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (workshop_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (workshopallocation_scheduled_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (quiz_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (quiz_statistics_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (chat_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (assign_cron). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (enrol_cohort). Please use scheduled tasks. ++
++ Use of legacy cron is deprecated (enrol_self). Please use scheduled tasks. ++ Use of legacy cron is deprecated (enrol_manual). Please use scheduled tasks. ++