Motivation for this change

Trying on a Rpi2 Jackett is crashing with an obscure memory violation exception when trying to add an indexer config.
on their repo README, it is stated that it requires mono 4.6+.

https://github.com/Jackett/Jackett#prerequisites

the current mono dependency targets 4.0+ which is insufficient
I tried using mono44 and mono46 but those failed to compile on my system.
I got a successful test using mono50.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.