install_and_upgrade.md: Use www.stackage.org/stack download links
install_and_upgrade.md: Use www.stackage.org/stack download links

narrative/stack,AndreasPK/stack,martin-kolinek/stack,mrkkrp/stack,deech/stack,juhp/stack,harendra-kumar/stack,Heather/stack,rubik/stack,harendra-kumar/stack,Fuuzetsu/stack,luigy/stack,supermario/stack,AndrewRademacher/stack,phadej/stack,Heather/stack,luigy/stack,Fuuzetsu/stack,keent/stack,supermario/stack,mathhun/stack,Blaisorblade/stack,achernyak/stack,sjakobi/stack,deech/stack,anton-dessiatov/stack,juhp/stack,keent/stack,lukexi/stack,Blaisorblade/stack,mathhun/stack,AndreasPK/stack,phadej/stack,lukexi/stack,fizruk/stack,vigoo/stack,MichielDerhaeg/stack,vigoo/stack,narrative/stack,AndrewRademacher/stack,fizruk/stack,anton-dessiatov/stack,rubik/stack,martin-kolinek/stack,sjakobi/stack,achernyak/stack,MichielDerhaeg/stack,rvion/stack,mrkkrp/stack,rvion/stack