Rebrand to Scheduled Tasks [com_scheduler]

[ditsuke]

Summary of Changes

Rename the component, the system plugin, language strings to match,

TODO

• [x] Rename component to com_scheduler => 'Scheduled Tasks'.
• [x] Update namespacing: Joomla\Component\Cronjobs => Joomla\Component\Scheduler.
• [x] Update component link refs.
• [x] Refactor component classes.
• [x] Refactor Field classes, update usages.
• [x] Update manifest.
• [x] Rename `#cronjobs` => `#scheduler_tasks`.
• [x] Update `#extensions` and `#assets` entries.
• [x] Update toolbars (task parameters).
• [x] Refactor model state (sets and gets).
• [x] Refactor asset privilege checks and form(s).
• [x] Rename system plugin to schedulerunner => 'Schedule Runner'.
• [x] Rename Cronjob(s) => Task(s).
• [x] Job plugin group => Task plugin group.
• [x] Rename events, refactor CronRunEvent class. ...

Testing Instructions

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Documentation Changes Required