server: notifications: send prune notifications via notification system

server: notifications: send prune notifications via notification system

If the `notification-mode` parameter is set to `legacy-sendmail`, then
we still use the new infrastructure, but don't consider the
notification config and use a hard-coded sendmail endpoint directly.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Tested-by: Gabriel Goller <g.goller@proxmox.com>
Reviewed-by: Gabriel Goller <g.goller@proxmox.com>
Tested-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>