refactor: move host-crontab generation into cron plugin

Host-crontab generation for `app.json` cron tasks now lives in the `cron` plugin, gated by a new `scheduler-uses-host-cron` trigger that the `docker-local` scheduler answers true while self-managed schedulers such as `k3s` answer false. This lets any host-cron scheduler participate in normal `app.json` cron without coupling to `scheduler-docker-local` or duplicating the crontab writer, while the `k3s` scheduler continues to manage its own in-cluster cron jobs.

Closes #8862.

plugins/scheduler-null/scheduler-uses-host-cron

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+trigger-scheduler-null-scheduler-uses-host-cron() {
+  declare desc="reports whether the scheduler writes cron tasks to the host crontab"
+  declare trigger="scheduler-uses-host-cron"
+  declare DOKKU_SCHEDULER="$1"
+
+  if [[ "$DOKKU_SCHEDULER" != "null" ]]; then
+    return
+  fi
+
+  echo "false"
+}
+
+trigger-scheduler-null-scheduler-uses-host-cron "$@"