net-mgmt/netbox: Update pkg-message

* Improve the user experience for upgrades from 2.11.x to 3.x by giving                                                                 more accurate info to enable the sample housekeeping task script. [1]                                                                 The instructions were taken from the NetBox page of the FreeBSD wiki.

* While I'm here: Relax some dependencies for upcoming updates.

Suggested by:	Mike Geiger (via e-mail) [1]