cephadm: change shared_folder directory for prometheus and grafana

cephadm: change shared_folder directory for prometheus and grafana

After https://github.com/ceph/ceph/pull/44059 the monitoring/prometheus
and monitoring/grafana/dashboards directories are changed to
monitoring/ceph-mixins. That broke the shared_folders in the cephadm
bootstrap script.

Changed all the instances of monitoring/prometheus and
monitoring/grafana/dashboards to monitoring/ceph-mixins

Also, renaming all the instances of prometheus_alerts.yaml to
prometheus_alerts.yml.

Fixes: https://tracker.ceph.com/issues/54176
Signed-off-by: Nizamudeen A <nia@redhat.com>