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]