"Prepare submissions for annotation" task tries to process the whole queue in one go

The '\assignfeedback_editpdf\task\convert_submissions' scheduled task has no limit when selecting records from the mdl_assignfeedback_editpdf_queue table
https://github.com/moodle/moodle/blob/589724236163476bdcf2310c2542b992dbfb9c7b/mod/assign/feedback/editpdf/classes/task/convert_submissions.php#L57

The task basically tries to process the whole queue in one go.

When there are lots of records in the queue, the tasks runs for many hours and then eventually dies or gets terminated. It seems like having a limit here will give admins more control under the task.