feat: add recurring cleanup job to generator

Its not immediately clear that finished jobs need to be cleaned up or
that solid queue has a built in way to do that.

This adds a commented out implementation of that recurring behavior to
the generator to make is easier for users to learn about the behavior.

Author: elasticspoon

lib/generators/solid_queue/install/templates/config/recurring.yml

@@ -8,3 +8,10 @@
 #     command: "SoftDeletedRecord.due.delete_all"
 #     priority: 2
 #     schedule: at 5am every day
+#
+#   Right now, there's no automatic cleanup of finished jobs.
+#   This reccuring job will clean up jobs older than the
+#   `clear_finished_jobs_after` period which defaults to 1 day.
+#   finished_job_cleanup:
+#     command: "SolidQueue::Job.clear_finished_in_batches"
+#     schedule: at 2am every day