net-p2p/jackett: update to 0.21.1239
- depend on mono6.8 explicitly and drop USES=mono
Changelog: https://github.com/Jackett/Jackett/releases
PR: 275150
Reported by: fernape
MFH: 2023Q4
(cherry picked from commit 2390530de4d1c58a5b15de03533c7fffbdb6ce34)