Upgrade information about FreeNAS and remove references to TrueOS

TrueOS was discontinued[1] and FreeNAS was merged into TrueNAS[2].

[1] https://www.truenas.com/trueos-discontinuation/
[2] https://www.ixsystems.com/blog/freenas-truenas-unification/

PR: 258590
Repported by: Graham Perrin <grahamperrin@gmail.com>