bug#60299: 'IceCat Support' link non-existent in Settings menu

[Adam Faiz via bug-gnuzilla via GNUzilla bug reports]

The 'IceCat Support' link in about:preferences links to 
https://libreplanet.org/wiki/Group:IceCat/preferences which doesn't exist.
I think the link should point to 
https://www.gnu.org/software/gnuzilla/#lists instead.