update rt.cpan.org URLs

The old ones will probably work indefinitely[1], but it would seem that they're officially deprecated[2].

[1] bestpractical/rt-extension-rt_cpan_org@17b7571d6d610e279257ac378d256e72bdd8514d
[2] bestpractical/rt-extension-rt_cpan_org@2630ea4bd54f9915c6185a9845ca234073ebd3c8