cronjobs: remove scheduling once and priority, fixes #4078

RELEASE-NOTES.md

app/controllers/admin/cronjobs/schedules.php

app/views/admin/cronjobs/schedules/display.php

app/views/admin/cronjobs/schedules/edit.php

app/views/admin/cronjobs/schedules/index.php

db/migrations/1.231_add_files_search_index.php

...ns/6.0.1_remove_cronjobs_scheduling_once_and_priority.php

lib/classes/CronjobScheduler.class.php

lib/models/CronjobSchedule.class.php

lib/models/CronjobTask.class.php

tests/unit/lib/classes/CronjobScheduleTest.php